Dissected ‘pleurembolic’ proboscis. Only the basal part inverts to form a sac (1 to 1) for the rest of the proboscis (2 to 2) to be retracted into without inversion. The radula, buccal mass and true mouth are in the distal end (3)  of the proboscis. White muscles attach to its proximal end (4). See also images  24 Tritia reticulata    & 25 Tritia reticulata .