I have seen this question a few times in person and on various forums. For at least one version prior to AutoCAD 2011, AutoCAD arcs included directional arrow grips on the ends that performed as Lengthen grips.
AutoCAD 2011 introduced multifunctional grips that allow for (you guessed it) multiple functions, and the arrows disappeared.
If you want to restore the arrow grips, you can set GRIPMULTIFUNCTIONAL to 0
