The flagellate Colponema[?] is also placed here by most workers. Some molecular studies have suggested that the Haplosporidia[?], a group of parasites on mostly marine invertebrates, might belong here too, but this is not supported by ultrastructure.
The main characteristic shared throughout the group is the presence of cortical alveoli, abutting sacs that form a continuous layer underneath the cell membrane and help support it. In ciliates, for instance, the alveoli form a flexible pellicle, and in dinoflagellates they often give rise to armor plates. Alveolates also characteristically have mitochondria with tubular cristae and flagella, when present, are ultrastructurally distinct.
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