Who is the muse in Homer's The Odyssey?

The Odyssey begins with an invocation of the Muse, one of the nine daughters of Zeus, who are associated with artistic inspiration.  It's sort of like a prayer in which the poet asks the Muse to either help him tell the story or to grant him divine inspiration so that he can eloquently tell the story of a great hero; sometimes the poet even asks the Muse to speak directly through him.  Each Muse is responsible for a certain area of art -- one is responsible for history, another for music and song, another for love poetry, etc. -- but the Muse one calls when writing an epic poem is Calliope because epic poetry is her sphere.  This invocation, or calling to the Muse, is considered to be a convention of epic poetry because it is typical for the poet to request such assistance at the beginning of epic poems.