Assertion: gastropod's head | /r/HasProperty | withdrawn into the shell
Facets:
value: by a powerful muscle
type: CAUSE
value: in snails
type: OTHER
Source sentences:
text: When the head and foot are withdrawn into the shell, the operculum follows to form a tight cover over the shell's opening.
text: In snails, the head and foot are withdrawn into the shell by a powerful muscle that is attached inside to the columella.