The painting relates to the story of the nymph Galatea which Ovid had borrowed from the Sicilian poet Theocritus and narrated in his Metamorphoses (13:750-768). Galatea was in love with the young Acis but the Cyclop Polyphemus (depicted in the adjacent lunette by Sebastiano del Piombo), jealous of the youth, killed Acis by throwing at him a piece of rock. The blood flowing from beneath the rock turned rapidly into a clear stream ending into the sea, the realm of Galatea. The river Acis still runs nowadays near the volcanic mountain of Aetna.
Galatea appears standing on a giant seashell led by two dolphins, surrounded by a company of nymphs and tritons while three putti in the air are drawing their bows. (Text: royalacademy.org.uk)

Source: Smarthistory