Source: http://wn.com/music_box
The music box consists of a set of typically small metal bars, structurally similar to those of the xylophone or kalimba. Instead of being hammered or struck as in the case of these instruments, they are "plucked" mechanically by small rods installed on a revolving cylinder. The position of these rods follow the score in a similar fashion as piano rolls.
Music box vary in size form small snuff boxes to large pieces of furniture.
Source: http://wn.com/music_box
http://www.youtube.com/watch?v=cgcdEk3LWzM