Commensal
(Com*men"sal) n. [LL. commensalis; L. com- + mensa table: cf. F. commensal. Cf. Mensal.]

1. One who eats at the same table. [Obs.]

2. (Zoöl.) An animal, not truly parasitic, which lives in, with, or on, another, partaking usually of the same food. Both species may be benefited by the association.

Believe it or not, I recently on Internet read that some
patients are more comfortable with a chosen intestinal parasite than without any.

The classic example is the termite. Termites don't digest wood. They do however have an organism living in their gut that does, so the termite gets nutrition from the commensal and shares nutrition with the bug in turn.

In fact (though I can't quote a source for this), humans have so many commensals, not just parasites, that the majority of DNA in the human body is not the DNA in the nuclei of your cells. Gut flora, famously, assist us in our digestive processes, which is one of the reasons why anti-biotics, being slightly indiscriminate in their actions, result in uncomfortable side effects - they decimate (literally take a tenth?) the gut flora as well.

and mitochondria? do they qualify?

Good question, etaoin. I haven't read much about them, but it does seem as though they started out that way.

Eta

Here is where the 'idea' of a commensal becomes fuzzy, and it's why people like me get all excitable and philosophically rigorous when we get to show off why we think Plato's theory of Forms is a pile of paapecack (or tutti as the kids in Bombay would say).

Our best guess (as I'm sure you know), is that mitochondria were part of the process of developing eukaryotes - cells with well-defined nuclei, as opposed to the prokaryotic cells of the bacteria and archaea. So mitochondria were originally bacteria, perhaps initially parasitic upon larger prokaryotic cells. In time, though, mitochondria became 'commensals' within cells. In further time (only about a billion years), mitochondria have become an integral part of the eukaryote package.

Or have they?

As you point out, they have their own DNA. They do not engage in sexual reproduction. They have their own genetic diseases. They have their own lineages, distinct from 'our' own. So are they parasites? Commensals (which would in general be taken to include symbiotes)? Parts of ourselves? Stick that in your cave of shadows and tell us, Mr Plato.

cheer

the sunshine "nominalism rocks" warrior

ah, Shanks, you give me far too much credit.
my knowledge of mitochondria does not extend much further than what I gleaned from Madeline L'Engle's Wrinkle in Time stories... though I have run across it since those, perhaps in something by Lewis Thomas? or Stephen Jay Gould? they seem to be a fascinating study of co-creation, and I truly appreciate your perspective and sharing and wit on the subject. I'm looking forward to learning more.

mitochondria--as shanks point out, have their own DNA-- and--everyone inherits their mother's dna (the male sperm only caries 'human dna'--so mitochondria are very useful now for tracing 'family lines' and 'genetic history'

they 'change/mutate' less then human DNA, do changes/mutations can be used to trace histories of populations.. (there was a claim some year ago of 12 or so 'eve's'--that by tracing the changes back, its clear that there were very few original woman who's genitic liniage is still in existance.. what ever, the original number of 'eve's', only 12 still have desendents alive today--or so mitochondria dna suggests.

(history of the male side of humankind can be traced through genetic changes on the Y chromisome.)

Mitochondria certainly, and chloroplasts in plants too, possibly, have been proposed as once-independent entities incorporated over evolutionary time into multicellular "higher" organisms.

In another direction - even the brush border of intestinal (and other) epithelial/surface cells seems to have been tacked on as an afterthought (and improvement), as it were...