Complex sentence: The presence of a clausilium is in fact the reason for the common name "door snails", because all the snails in this family have a sliding "door" or clausilium, which can close the aperture of the shell.
Split into simpler sentences: The presence of a clausilium is in fact the reason for the common name "door snails". Clausiliids all have a sliding "door" or clausilium, which can close the aperture of the shell.