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Other Invertebrates: World's Weirdest: Flatworm Penis Fencing

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      • © National Geographic

        World's Weirdest: Freaks in the Ocean

        World's Weirdest: Flatworm Penis Fencing

        For flatworms, "Who's your daddy?" is a loaded question. In a bizarre bout lasting up to an hour, the first flatworm to stab and inseminate its mate becomes the father.

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