Excavata
| Excavata | |
|---|---|
 |
|
| Giardia lamblia, a parasitic diplomonad | |
| Scientific classification | |
| Domain: | Eukaryota |
| (unranked): |
Excavata (Cavalier-Smith), 2002 |
| Phyla | |
Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota. Introduced by Thomas Cavalier-Smith in 2002 as a new phylogenetic category, it contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans. Excavates were formerly considered to be included in the now obsolete Protista kingdom. They are classified based on their flagellar structures, and they are considered to be the oldest members (basal lineage) of flagellated organisms.
The Discobaean excavata (Euglenozoa, Percolozoa, Tsukubea, Jakobea) now appear to be basal Eukaryotes, while the Metamonadaean and Malawimonaen excavata now appear to be sister clades of the Podiata. The Discoba may be highly paraphyletic.
Many excavates lack 'classical' —these organisms are often referred to as 'amitochondriate', although most retain a mitochondrial organelle in greatly modified form (e.g. a hydrogenosome or mitosome). Among those with mitochondria, the mitochondrial cristae may be tubular, discoidal, or in some cases, laminar. Most excavates have two, four, or more flagella and many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules. However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences).
The closest that the excavates come to multicellularity are the Acrasidae slime molds. Like other cellular slime molds, they live most of their life as single cells, but will sometimes assemble into a larger cluster.
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