Table 2. Comparison of the key features of gregarine attachment organelles.

	Mucron of archigregarines (Selenidium)	“Mucron” of aseptate eugregarines	Epimerite of septate eugregarines
Shape	Knob-like	Sucker-shaped or dome-shaped	Various, usually—a well-developed frontal protuberance of the cell of diverse shape
Tegument structure in the region of the junction with the host cell	The tegument of mucron is trimembrane pellicle excepting small region in front of conoid, where a cytostome and duct of mucron vacuole is intermittently formed, IMC is absent and there is just a single plasma membrane	The IMC of the pellicle terminates at the edge of the cell junction zone, so the tegument of the attachment organelle is represented only by a single plasma membrane	The IMC of the pellicle terminates at the edge of the cell junction zone, so the tegument of the attachment organelle is represented only by a single plasma membrane
Cell junction between host and parasite	Septate cell junction; no peculiar structures on the edge of the junction zone	Two closely adjacent plasma membranes (of host and parasite) forming high electron density zone. A circular groove in the gregarine tegument (plasma membrane) runs along the edge of the region of the cell junction (where the IMC terminates) and pinches a small portion of the host cell	Two closely adjacent plasma membranes (of host and parasite) forming high electron density zone. A circular groove in the gregarine tegument (plasma membrane) runs along the edge of the region of the cell junction (where the IMC terminates) and pinches a small portion of the host cell
Cytoplasm organelles	Apical complex (conoid, apical polar ring(s), rhoptries) and mucronal (food) vacuole	Frontal region of “mucron” contains fibrillar zone or large vacuole with fibrillar content adjoining the cell junction zone; no mitochondria were observed; longitudinal actin-like fibrillar structures (filaments) are well developed	Frontal region of epimerite contains a large flattened frontal vacuole with fibrillar content adjoining the cell junction zone; it is built from ER vesicles; mitochondria, often numerous and arranged in a layer, are located beneath this vacuole during growth and development of the trophozoite; different inclusions (lipid globules, amylopectin granules); longitudinal fibrillar structures (microfilaments and microtubules) can be well developed within the stalk of epimerite (if present)
Functioning	Attachment and feeding by myzocytosis: formation of temporary cytostome-cytopharingeal complex consisting of mucronal vacuole with the duct running through the conoid	Attachment; feeding is questionable, no myzocytosis observed	Attachment; feeding is questionable, no myzocytosis observed
Fate	Mucron of archigregarines persists for long time after trophozoite detachment and retains apical complex (conoid at least) till (including) stage of syzygy	Unknown; most likely is to retract/condense: frontal part of mature detached gamonts is covered by trimembrane pellicle, but not by a single plasma membrane	When trophozoite transforms into mature gamont, the epimerite is to break off or to retract/condense
Studied species	Selenidium pendula, S. hollandei, S. orientale, S. pennatum	Lecudina sp. from Cirriformia tentaculata, L. pellucida, Lankesteria levinei, Difficilina cerebratuli	Didymophyes gigantea, Epicavus araeoceri, Gregarina spp., Leidyana ephestiae, Pyxinia firmus, Stylocephalus africanus
References	Schrével (1968), Schrével (1971a), Kuvardina & Simdyanov (2002), Simdyanov & Kuvardina (2007), Schrével et al. (2016)	Schrével & Vivier (1966), Ouassi & Porchet-Henneré (1978), Simdyanov (1995b), Simdyanov (2009)	Grassé (1953), Devauchelle (1968), Baudoin (1969), Desportes (1969), Ormierès & Daumal (1970), Hildebrand (1976), Ormierès (1977), Tronchin & Schrével (1977), Marquès (1979), Ghazali et al. (1989), Valigurová & Koudela (2005), Valigurová et al. (2007), Valigurová, Michalková & Koudela (2009), Schrével et al. (2013)