"This group represents a separate case of worm-snails having lost the operculum [a plate that closes the aperture of a mollusk's shell] and developed other means of protection from predation ...

Worm snails also do not have a protective covering found in other snails called an operculum. This body part allows the snails to retreat further inside their shell and keep their bodies moist.

using a special “lid” (the operculum) to seal their shells and avoid drying out while they wait out the dry periods buried in ...

The olive-colored critters — which grow to three inches long — are identifiable by their coiled shell and “operculum (”trapdoor”) covering the opening, which is missing when the snail is ...