Normally its neck is out and exposed so that it can gather food. If something were to threaten, it would withdraw back into his burrowed shell.

The shell of the Scale-sided Piddock is in overlapping plates, the rear end of which has groved ridges intended to wear down sand or soft rock to create a burrow for safety.

Sometimes, as here, the neck is larger than the opening in the end of its shell. This can provide greater protection when the neck is withdrawn, because the opening is even smaller than normal.