Cancer : Myths and Legends
Cancer is the 4th sign of the zodiac, represented by the Greeks as a Crab. It is the enormous creature which the goddess Hera sent from the swamp against Herakles during his battle with the Lernaean Hydra, his second labour. The crab nipped his foot and he stamped on it, crushing it beneath his heel, whereupon Hera set it in the zodiac as one of the twelve signs.
The story is a strange echo of that involving the other zodiacal creature with claws - Scorpio. The Greek year began in Cancer, on the new moon after the Summer Solstice, and the story is of each new year following on the heels of the old.
The Mythology: The most widely accepted story is that Cancer was sent by the goddess Hera to attack Hercules while he was battling with the Hydra on the second of his labours. During the struggle, the crab was crushed beneath the feet of Hercules. As a reward for its heroic, but futile, efforts, Hera placed the crab among the stars.