Arethusa, a fountain in the island of Orty-gia, which formed a part of the ancient Syracuse, in Sicily. The ancients supposed its waters to be united with those of the river Al-pheus in Peloponnesus. The naiad of the fountain was the nereid Arethusa. (See Alpheus.) - Another Arethusa was one of the Hesperides, the guardians of the golden apples, to obtain which was one of the 12 labors of Hercules.