How does hypodermic impregnation occur in some free-living flatworms?

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In some free-living flatworms, hypodermic impregnation is a specialized form of copulation. During this process, the penis of one individual pierces the epidermis of another flatworm and injects sperm directly into its tissues.1

While most flatworms possess complex reproductive systems with male and female ducts for standard copulation, those species that lack a female duct rely on this method of injection, with fertilization subsequently occurring within the inner layer of tissue.2