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We studied and reviewed the morphology, ontogenesis and encystment of two closely related spathidiids, Arcuospathidium cultriforme (Penard, 1922) Foissner, 1984 and A. scalpriforme (Kahl, 1930) Foissner, 2003, using live observation, protargol impregnation, morphometry, scanning electron microscopy, populations from different geographic regions, and literature data. Both species are biogeographic flagships. They are 200 300 μm long and have a long, steep oral bulge. Our investigations show that they are very similar, differing mainly in the length of the oral bulge and the arrangement of the oral bulge extrusomes. Thus, we classify them as subspecies within the Arcuospathidium cultriforme complex, which includes the following taxa: A. cultriforme cultriforme (Penard, 1922), A. cultriforme scalpriforme (Kahl, 1930), A. cultriforme megastoma Foissner et al., 2002, and A. lorjeae Foissner et al., 2002. The latter taxa are likely to have a restricted geographic distribution, and there is evidence that the South American and Rwandan populations of A. cultriforme cultriforme could represent further distinct (sub)species. During encystment of A. cultriforme, the macronucleus becomes strongly shortened and the infraciliature appears to be resorbed. The mature cyst is unique in having a very thick, faceted wall. A detailed scenario of the ontogenetic processes is provided, showing that the ontogenesis of A. cultriforme matches those of haptorids in general and other spathidiids in particular. However, there are several characteristic features, for some of which we could find reasonable explanations which, in turn, provided models for the spathidiid ontogenesis and evolution: (i) the outgrowth model suggests that the slope of the long spathidiid oral bulge is obtained mainly by faster growth of the dorsal than the ventral side; (ii) the row detachment model proposes that the Arcuospathidium and Epispathidium ciliary patterns are variations of the Spathidium pattern which evolved from the Protospathidium pattern; and (iii) a model is proposed for a Dileptus like ancestor of the spathidiids.