At the beginning Scilla was a nymph, daughter of Ceto and Forco. Scilla lived in Sicily and used to go on Zancle beach and swim in the sea. One evening, near the beach, she saw Glauco, son of Poseidon, appear from the waves of the sea. He was a god of the sea half man and half fish.

Scilla, terrified, took refuge on the summit of a mountain near the beach. The god, surprised by the reaction of the nymph, began to scream his love to her, but Scilla escaped leaving him alone in his pain. Then Glauco went to the sorceress Circe and asked her for a love potion to make him fall in love with the nymph, but Circe, wishing god for her, asked him to join her. Glauco refused to betray his love for Scilla and Circe, furious, wanted revenge.

When Glauco was gone, Circe prepared a potion and went to the beach Zancle, towards the potion to the sea and returned to her home. When Scilla arrived and dived into the water, she saw growing around her monstrous heads of dogs. Frightened, she escaped from the water but she realized that the dogs were attached to her legs with a serpentine neck. She realized that until the basin she was still a nymph, but instead of legs, sticking out six fierce faces of dog. Terrified, Scilla jumped into the water and went to live in the hollow of a rock near the cave where lived also Cariddi.

In principle, Cariddi was a naiad, daughter of Poseidon and Gea, devoted to the robberies and famous for her voracity. One day she stole the oxen of Gerione to Heracles and ate some. Then Zeus, thundered her, letting her fall into the sea, where she turned into a monster like a lamprey, which formed an ocean vortex with its huge mouth, able to swallow the passing ships.

The legend sets Cariddi at one of the two sides of the Strait of Messina, opposite to the cavern of the monster Scilla. The ships that feed the strait were forced to spend close to one of two monsters.

Source Wikipedia