The term (like basileus or tyrannos) is probably of Pre-Greek etymology[1] (possibly cognate to Etruscan (e)prθni).
They ate at public expense in the tholos, a circular edifice constructed for them next to the boule house.
Each day, for one 24-hour period, one member of the 50 prytaneis was selected by lot to serve as the foreman (ἐπιστάτης epistates, "caretaker").
No man was allowed to hold this office more than once, and so probably more than half of all adult male Athenians held it, at one time or another.
In the Fourth Century, this practice changed and the chairmanship of meetings was taken over by an office specifically created for this task (the πρόεδροι proedroi).