The 9 Greek Muses

At one time, the Muses were anthropomorphic goddesses, possibly of prophetic springs, who became the representatives of poetry, the arts and science, and sources of inspiration. They sang, like the bird-bodied Sirens with whom they are sometimes contrasted. Homer refers to them as one Muse and as many Muses, living on Olympus. Plato lists eight muses connected with eight mythical spheres. Hesiod refers to them as 9 daughters of Zeus and Mnemosyne, who were born in Pieria, which is described as "watered by the springs flowing from Olympus," according to "Muses and Sirens," by J. R. T. Pollard; The Classical Review New Series, Vol. 2, No. 2 (Jun., 1952), pp. 60-63.

"(ll. 53-74) Them in Pieria did Mnemosyne (Memory), who reigns over the hills of Eleuther, bear of union with the father, the son of Cronos, a forgetting of ills and a rest from sorrow. For nine nights did wise Zeus lie with her, entering her holy bed remote from the immortals. And when a year was passed and the seasons came round as the months waned, and many days were accomplished, she bare nine daughters, all of one mind, whose hearts are set upon song and their spirit free from care, a little way from the topmost peak of snowy Olympus.
Hesiod Theogony