Excavates
Giardia lamblia, a parasitic diplomonad
Scientific classification
Domain: Eukaryota Kingdom: Excavata (Cavalier-Smith) Simpson, 2003
Phyla
Metamonada Loukozoa Euglenozoa Percolozoa

The excavates are a major assemblage of protists, often known as Excavata. Giardia lamblia (synonymous with Lamblia intestinalis and Giardia duodenalis) is a Flagellated Protozoan Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The metamonads are a large group of Flagellate protozoa Their composition is not entirely settled but they include the Retortamonads Diplomonads and The Euglenozoa are a large group of Flagellate protozoa They include a variety of common free-living species as well as a few important parasites some of which infect humans The Percolozoa are a group of colourless Protozoa, including many that can transform between Amoeboid, Flagellate, and encysted stages Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this The phylogenetic category Excavata contains a variety of free-living and symbiotic forms, and includes some important parasites of humans. Many excavates lack 'classical' mitochondria - these organisms are often referred to as 'amitochondriate', although, most, perhaps all, retain a mitochondrial organelle in greatly modified form. Others have mitochondria with tubular, discoidal, or in some cases, flat cristae. Cristae (singular crista) are the internal compartments formed by the inner membrane of a Mitochondrion. Most excavates have two, four, or more flagella and many have a conspicuous ventral feeding groove with a characteristic ultrastructure, supported by microtubules. A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it Ultrastructure (or ultra-structure is the detailed structure of a biological specimen such as a cell, tissue, or organ, that can be observed by Electron Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers However, various groups that lack these traits may be considered excavates based on genetic evidence (primarily phylogenetic trees of molecular sequences). Most excavates fall into one of four groups, which may be treated as phyla:

Of these, the Percolozoa (Heterolobosea) and Euglenozoa appear to be particularly close relatives, and are united by the presence of discoid cristae within the mitochondria. Euglena are a common group of Unicellular Protists, of the class Euglenoidea of the Phylum Euglenophyta. Trypanosoma are of the class kinetoplastida a Monophyletic group of unicellular parasitic Protozoa. Cristae (singular crista) are the internal compartments formed by the inner membrane of a Mitochondrion. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Most jakobids have tubular cristae, like most other protists, while the metamonads are unusual in having lost classical mitochondria - instead they have 'hydrogenosomes', 'mitosomes' or uncharacterised organelles. Excavate relationships are still uncertain; it is possible that they are not a monophyletic group. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor

Certain excavates are often considered among the most primitive eukaryotes, based partly on their placement in many evolutionary trees. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex This could encourage proposals that excavates are a paraphyletic grade that includes the ancestors of other living eukaryotes. In Phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all However, the placement of certain excavates as 'early branches' may be an analysis artifact caused by long branch attraction, as has been seen with some other groups, for example, microsporidia. Long branch attraction (LBA is a phenomenon in Phylogenetic analyses (most commonly those employing Maximum parsimony) when rapidly evolving lineages are inferred

The monophyly of the excavates is far from clear, although it seems like there are several clades within the excavates which are monophyletic. ^[1]

References

1. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  

• Cavalier-Smith, T. (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology 52: 297–354.  
• Alastair G. B. Simpson (2003). "Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata". International Journal of Systematic and Evolutionary Microbiology 53: 1759–1777. doi:10.1099/ijs.0.02578-0. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.