The alveolates include several significant and diverse groups of Protista whose relationships to one another have only recently been recognized on the basis of ultrastructural and molecular studies. The main lines of alveolates are the following:
- The Ciliophora, including many common protozoa: Paramecium, Stentor, etc.
- The Apicomplexa, including mostly parasitic forms
- The Dinoflagellates, including mostly marine zoo- and phytoflagellates
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.