The alveolates (meaning "pitted like a honeycomb")[2] are a group of protists, considered a major clade[3] and superphylum[4] within Eukarya.

These are flattened vesicles (sacs) arranged as a layer just under the membrane and supporting it, typically contributing to a flexible pellicle (thin skin).

[8][9] The relationship of apicomplexa, dinoflagellates and ciliates had been suggested during the 1980s, and this was confirmed in the early 1990s by comparisons of ribosomal RNA sequences, most notably by Gajadhar et al.[10] Cavalier-Smith introduced the formal name Alveolata in 1991,[11] although at the time he considered the grouping to be a paraphyletic assemblage.

In 2001, direct amplification of the rRNA gene in marine picoplankton samples revealed the presence of two novel alveolate lineages, called group I and II.

Some studies suggested the haplosporids, mostly parasites of marine invertebrates, might belong here, but they lack alveoli and are now placed among the Cercozoa.

Silberman et al 2004 establish that the Thalassomyces genus of ellobiopsids are alveolates using phylogenetic analysis, however as of 2016[update] no more certainty exists on their place.

Cavalier-Smith proposed the alveolates developed from a chloroplast-containing ancestor, which also gave rise to the Chromista (the chromalveolate hypothesis).

Other researchers have speculated that the alveolates originally lacked plastids and possibly the dinoflagellates and Apicomplexa acquired them separately.

In other words, the term Myzozoa, meaning "to siphon the contents from prey", may be applied informally to the common ancestor of the subset of alveolates that are neither ciliates nor colponemids.

In one school of thought the common ancestor of the dinoflagellates, apicomplexans, Colpodella, Chromerida, and Voromonas was a myzocytotic predator with two heterodynamic flagella, micropores, trichocysts, rhoptries, micronemes, a polar ring and a coiled open sided conoid.