Janus, a two-headed Greek tortoise, celebrated his tenth birthday on Wednesday at the Natural History Museum in Geneva. The museum also invited children born in 1997 to join the party, offering cake, presents, and a documentary film screening.

The tortoise—named after the Roman god of doors, beginnings, and endings—is the result of the same genetic anomaly that gives rise to conjoined twins in humans. Two embryos began to develop, but the process stopped partway, leaving both heads attached to one body.

Such creatures rarely survive in the wild, experts say, but can thrive in captivity.

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