Cancer is not actually ruled by a planet, it is ruled by a “luminary” called the MOON. The only other luminary is the sun! The term planet is a relative term. The crescent moon symbol means mind or evolving human spirit (crescent).
In the myth, Hera sends the crab to distract Hercules while the hero is fighting the Lernaean Hydra, the serpent-like beast with many heads and a poisonous breath of fire. Each time Hercules would cut off a head, two more would grow in its place. When the crab tries to kill Hercules, or at least distract him long enough that the hydra kills him, Hercules kicks it all the way to the stars. This happened during one of the “twelve labors” that was put upon Hercules for crimes that Hercules committed while under the influence of a spell that queen Hera placed upon the hero. When the crab was extinguished, Hera placed the crab among the stars for eternity.