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Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly- (Greek: “πολύ”) meaning “multiple” and kephalē- (Greek: “κεφάλη”) meaning “head”. A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.
Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.
In humans, there are two different forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus.
There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.