ABSTRACTThis paper describes the cortical anatomy and development of mirror‐image doublets ofStylonychia mytilus, analyzed using the protargol technique. The reversed, or “left‐handed” (LH) component of these doublets is a mirror image of the normal or “right handed” (RH) component with regard to the arrangement of cortical structures. The mirror‐image patterning is imperfect, however, as the individual ciliary structures of the LH component all are of normal internal asymmetry, and the orientation of membranelles is inverted. Certain structures that would be expected to form near the line of symmetry are absent. During cell division and cortical reorganization, ciliary primordia arise and become arranged in a mirror‐image pattern that is more perfect than that exhibited by the mature structures. Deviations from a mirror‐image pattern appear at late stages when organelle sets differentiate within ciliary primordia: for example, the membranelle set differentiates within the oral primordium of the LH component in a sequence that is an inversion rather than a mirror image of the corresponding sequence of the RH component. This mixed control of oral development by different cortical “informational systems” accounts for some of the characteristic abnormalities of the mature oral structure