SYNOPSIS.Bresslaua voraxis a dimorphic ciliate with micro‐stome and macrostome forms. The development of the macro‐stome form is associated with pronounced extension of the vestibule and vestibular opening and gross torsion of the ciliate which finally shows little resemblance to the microstome form. These transformations culminate a phylogenetic trend in the Colpodidae which can be followed from the ciliates of relatively simple structure, such asWoodruffia, throughBryophrya, ColpodaandTillinatoBresslaua.This phylogenetic trend is marked by the increasing importance of stomatogenesis. The morphogenetic pattern of these transformations appears to distinguish the Colpodidae from other trichostomatous ciliates, such as the Plagiopyli