Polycephaly

Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly (Greek: "πολύ") meaning "many" and kephalē (Greek: "κεφαλή") 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 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. Two-headed people and animals, though rare, have long been known to exist and documented. In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly. In dicephalus parapagus dipus, the two heads are side by side, on a torso with two legs, with varying levels of twinning of organs and structures within the torso. The shared body may have four arms altogether, or three arms, or two arms only. There are Greek-based medical terms for the variations, e.g. dibrachius means two-armed, tribrachius means three-armed. Both heads may contain a fully formed brain, or one may be anencephalic. If carried to term, dicephalus parapagus twins are usually stillborn, or die soon after birth.