ABSTRACTA hypothesis is presented to explain the mechanism of action of the gun cell ofHaptoglossa mirabilis.It is suggested that the gun cell is activated by mechanical breaking of a line of weakness which dislodges the plug in the bore. The tubular system inside the cell everts, like the finger of a glove, from the fixed end at the muzzle. The basal vacuole takes in water and expands pushing the protoplasmic contents of the cell up the everted tube. The apical portion of the everted tube, containing reserve wall material, expands to accommodate the protoplasmic contents of the cell. The swollen apical portion containing the protoplasmic contents of the gun cell breaks away from the everted tube to form the infective sporidium inside the host animal.