These flatworms plunge their penises into their own heads to inject themselves with sperm (when they must).

The flatworm Macrostomum hystrix isn’t exciting to look at. Its diet of microalgae doesn’t raise any eyebrows, and you probably wouldn’t even notice one if you came across it in its native habitat. But in the bedroom, these flatworms take kink to a whole new level: when they can’t find a partner, they will stab themselves in the head with their needle-like penises and inject sperm to self-fertilize.

In principle, it is what scientists call hypodermic insemination: the practice of forceful

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