The parabasalids are a group of flagellated protist
s within the supergroup Excavata
. Most of these eukaryotic organism
s form a symbiotic
relationship in animal
s. These include a variety of forms found in the intestines of termite
es, many of which have symbiotic bacteria
that help them digest cellulose
in woody plant
s. Other species within this supergroup are known parasite
s, and include human pathogen
The flagella are arranged in one or more clusters near the anterior of the cell. Their basal bodies
are linked to parabasal fiber
s that attach to prominent Golgi complexes
, distinctive to the group. Usually they also give rise to a sheet of cross-like microtubule
s that runs down the center of the cell and in some cases projects past the end. This is called the axostyle
, but is different in structure from the axostyles of oxymonad
Parabasalids are anaerobic
, and lack mitochondria
, but this is now known to be a result of secondary loss, and they contain small hydrogenosome
s which apparently developed from reduced mitochondria. Similar relics have been found in other amitochondriate flagellates, and the parabasalids are probably related to them, forming a group called the metamonad
s. They lack the feeding grooves found in most others, but this is probably a secondary loss as well.
Before reclassification, the parabasalids are currently divided into about 7 to 10 orders
depending on sources. Present classification divided Parabasalia into 4 orders, that is, Trichonymphida, Spirotrichonymphida, Cristamonadida, and Trichomonadida.
* The trichomonad
s have one group of 4-6 flagella, one of which is attached to the side of the cell and often forms an undulating membrane. Many are found in vertebrate hosts, including ''Trichomonas vaginalis
'', which causes a sexually transmitted disease in humans.
* The other orders, formerly grouped as the hypermastigid
s, have a large number of flagellar clusters and are found exclusively in the guts of insects. (The term "Hypermastigida" is still occasionally encountered.