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Freeze-fracture studies reveal that extrusive organelles displaying saltatory particle movements in centrohelidian axopod are attached to highly ordered domains within the plasma membrane. It is postulated that the motive force for lateral displacement of these membrane domains with the adhering organelle is located immediately underneath the plasma membrane being either part of the peripheral membrane proteins or attached filaments alined parallel to the axopodial micro­ tubules. The attachment domain is interpreted organelle discharge by membrane fusion.

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Reinvestigation of the type population of the sorocarp‐forming ciliate Sorogena stoianovitchae Bradbury & Olive, 1980 using the Fernández‐Galiano technique and various electron‐microscopy techniques (scanning electron microscopy, freeze‐fracture and ultrathin sections) expands the observations reported in the original description of the species, Sorogena stoianovitchae is a colpodid ciliate with oral ciliature consisting of 25 ciliated paroral dikinetids on the right and 3‐5 small adoral organelles on the left of an elongated and domed oral slit, resembling that of the genus Platyophrya. Sorogena stoianovitchae divides in the free swimming condition and not in a division cyst, as is the case in the colpodids sensu stricto (s. str.), e.g. Colpoda, Bresslaua. or Tillina, As shown in a detailed light‐microscopy study, morphogenesis in S. stoianovitchae is of the stomatic mode typical for certain colpodid ciliates. Based on the wealth of new information the phylogenetic position of S. stoianovitchae is discussed at some length and arguments are given in favor of the following classifications: S. stoianovitchae Bradbury & Olive, 1980 currently sole member of the family Sorogenidae Bradbury & Olive, 1980; order Sorogenida Foissner, 1985;subclassColpodiaFoissner, 1985; class Colpodea Small & Lynn, 1981. This investigation facilitates the discovery of further members of this genus reported primarily from the tropical and subtropical zone.