Medusa was a mortal woman whom Athena changed into a Gorgon as punishment for desecrating her temple, sleeping with Poseidon there. Some say that Poseidon raped her, while others say that she willingly had interocurse with him in Athena's sanctuary. Nonetheless, when Athena became aware of these activities going on in her temple, she became enraged. It may have been jealousy that provoked Athena to turn Medusa from a beautiful woman into the gorgon, for Medusa was reputed to be beautiful, and her hair was particularly splendid. When Athena came upon Medusa and Poseidon (also an arch-rival of Athena's since he vied for dominance over Athens, Athena offering the olive tree, Poseidon, the horse), she turned Medusa's beautiful hair into snakey tendrils and banished her to the far ends of the earth beyond the Hyperborean lands where she remained with her sisters.
Meanwhile, the Agean king, Acrius, heard an oracle that told him that if his daughter Danae gave birth to a son, this grandson would kill him. In an effort to defy the oracle, he locked Danae up in a brazen tower. There Zeus came to her as a shower of gold, impregnating her. Incarcerated, Danae gave birth to a son, Perseus. When the king Acrius heard the clamor of the child, he realized that his efforts to imprison his daughter were in vain. He locked Danae and her new born son Perseus up in a wooden chest, and cast them into the sea.
Drifting in the sea, the fisherman, Dictys scooped them up with his net and took the mother and child to the island of Seriphos, where his brother Polydyctes reigned. There Perseus grew. Some say that Polydyctes wanted to marry Danae, and it was to save his mother from the marriage that Perseus offered to go off and slay Medusa, while others say that since all guests brought a horse as a marriage gift, Perseus, having no gift, offered to go off and bring Medusa as a fitting wedding gift for Polydyctes. For either reason, Perseus left the island of Seriphos and set off, intent to return with Medusa.
In biology, a medusa (also known as a hydromedusa) is a form of coelenterate[?] in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with the hydroid[?] or polyp. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the stomach and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles.
In the class Hydrozoa[?] medusae are the sexual individuals of many species, alternating in the life cycle with asexual polyps, but in Scyphozoa or (jellyfishes) proper the medusa alone is well developed.
Except the freshwater jellyfish, medusa do only appear in the life cycle of marine hydrozoa and scyphozoa.