These are two fountains that flank the equestrian statue of Ferdinando I at Piazza Santissima Annuziata. They are the work of Italian sculptor Pietro Tacca and were erected on the square in 1641. They are considered masterpiece works of Mannerism with a marine theme.
At each fountain you will see pairs of sea monsters, back to back, from which water pours downward from their mouths into elongated basins which creatively have upward tilting, creature-like mouths of their own. From these mouths spout upward flows of water also falling into the basins. It is a clever and beautiful fountain design that achieves balance and symmetry. Well worth spending a few minutes to enjoy if your are a fan of fountain and sculpture.