As the Udon noodles are shaped round, they are called 'tama,' literally meaning 'ball' (the term 'tama' is also used as a unit for measuring Udon noodles like 'hito-tama, futa-tama' [Udon noodles for one eater, Udon noodles for two eaters, in that order]).
丸くまとめるので「玉」と言われている（この「玉」という言葉はうどんの量の目安となる単位にも「1玉、2玉」などという表現で使われる）。