Acis, the son of the Italic god Faunus and the Nymph Symaethis, gained the love of the Nereid, or the sea nymph, Galatea. Galatea had another admirer, Cyclopus Polyphermus, who loved her without any hope for success. He played for Galatea love songs on his pipes. Once he found her lying in the arms of Acis. In his intense jealousy, Polyphermus tried to crash his rival under rocks, but Acis turned himself into river and thus escaped from the giant.
See: Nicolas Poussin. Acis and Galathea.