According to Greek mythology the giant Crab took part in a battle between a terrible the multi-headed Lernaean Hydra and Heracles. Hera, who had always hated Heracles, sent forth Crab to aid the serpent. He grabbing onto Hercules’ toes with its claws. But Heracles smashed the Crab with his foot after it pinched his toe. Hera, grateful for Crab’s heroic effort and courage, gave it a place in the sky.
Another interpretation of the Cancer constellation is that an enormous Crab called Crios was blessed with immortality, he protected Poseidon’s daughters, the sea nymphs. When Typhon was terrorising the Gods of Olympus, Crios sought help in protecting his brood in the form of a giant squid called Vamari. But Vamari or the Vampire Squid ate the missing sea nymphs. This led to a battle won by Crios, although he was badly hurt. Because he was immortal he had evermore to live in pain. Poseidon relieved him by placing him in the sky as Cancer.