The Vesica Piscis
The term "vesica piscis" is first recorded in literature in 1809, but is no doubt much older. It translates from Latin literally as "fish bladder," but more likely refers to any bladder that takes the form of a fish when filled (aside from the anatomical definition, a bladder can be anything that functions similarly: a sewn skin that holds wine, for example, or the air sack of bagpipes.) The Vesica Piscis symbol appears frequently in medieval art and architecture, and the symbol's roots go back further still.
The Vesica Piscis is made by linking two cicles together, bringing the outside edge of each to the midway point of the other. The almond-shaped center of the image is called a mandorla (Latin for almond; "vesica" or "vesica piscis" is sometimes also used to describe only this almond-shaped center.) The mandorla can easily be seen as a grail or chalice, connecting the symbol to Avalon.
When the Vesica Piscis is displayed vertically, the mandorla forms the shape of a fish. The word "fish" translates into Greek as "ichthys", which is an acronym for "Jesus Christ, Son of God, Savior." Early Christians adopted the fish symbol as their own, and used it as sort of secret code to identify themselves to one another and avoid persecution. As previously stated, the mandorla has been much used in art and achitecture: as a frame for Jesus and the saints, or as the passage between heaven and earth through which Jesus ascends. Far from a secret anymore, the icthys has become the preeminent icon of bumper evangelism, and spawned a whole slew of angry responses: little fish with feet that say "Darwin," an icthys with shark fins, ones that say "kosher," etc.
When the Vesica Piscis is viewed horizontally, however, the mandorla becomes a different sort of passage: the birth passage. The pointed oval is a universal symbol of the Divine Feminine, and in this context the Vesica Piscis is the vulva of the Goddess, surrounded by the crescents of the waxing and waning moon. The mandorla as birth passage can easily be seen on the sheila-na-gig figures found on Irish churches, and in the squatting figures of the Hindu goddess Kali. Almonds are a primeval fertility symbol, and as such are associated with the Phrygian goddess Cybele and her transgendered consort, Attis, and the Greek nymph Phyllis, who was metamorphosed into an almond tree. Fish, too, play a significant part in the lives of goddesses from many cultures. "Delphos" translates from Greek as both "womb" and "dolphin." The Chinese Great Mother Kwan -Yin often appears as a fish goddess, as does Aphrodite. Kali, as the swallower of Shiva's penis, becomes Minaksi, the fish-eyed one; Isis becomes Abtu, the Great Fish of the Abyss, when she swallows the penis of Osiris.
A medieval hymn calls Jesus "the little fish in the Virgin's fountain." The Christ child is often shown inside a mandorla, superimposed over Mary's womb. Mary herself can be equated with the goddess Aphrodite Marina, who brought forth all the fish in the oceans; Marina's blue robe and pearl necklace, like the Christian Mary's, are classic symbols of the sea. On Cyprus, Mary to this day is worshipped as "Panaghia Aphroditessa." The connections are many: the Vesica Piscis illumines the common heritage of Christianity and the Goddess traditions it absorbed, traditions it would later vilify and all but destroy.