The alveolates are a major line of protists. There are three main groups, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:

The most notable shared characteristic is the presence of cortical alveoli, flattened vesicles packed into a continuous layer supporting the membrane, typically forming a flexible pellicle. In dinoflagellates they often form armor plates. Alveolates have mitochondria with tubular cristae, and their flagella or cilia have a distinct structure.

The Apicomplexa and dinoflagellates are closer to each other than they are to the ciliates. Both have plastids, which likely have a common origin. Most also share a bundle or cone of microtubules at the top of the cell. In apicomplexans this forms part of a complex used to enter host cells, while in some colorless dinoflagellates it forms a peduncle used to ingest prey. Various other genera are closely related to these two groups, mostly flagellates with a similar apical structure. They include free-living members in Oxyrrhis, Acrocoelus, Colpodella, and perhaps Colponema, and parasites in Perkinsus, Parvilucifera, Cryptophagus, and the ellobiopsids.

Relationships between the major groups were suggested during the 1980s, and confirmed by a genetic study by Gajadhar et al. in 1991, after which the name Alveolata was introduced by Cavalier_Smith. Some studies suggested the haplosporids, mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa.

References

• Gajadhar, A. A. et al. (1991). Ribosomal RNA sequences of Sarcocystis muris, Theilera annulata, and Cryptothecodinium cohnii reveal evolutionary relationships among apicomplexans, dinoflagellates, and ciliates. Molecular and Biochemical Parasitology 45: 147-153.
• Cavalier-Smith, T. (1993). Kingdom Protozoa and its 18 Phyla. Microbiological Reviews 57:953-994. PubMed (http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid=372943)