These flatworms plunge their penises into their own heads to inject themselves with sperm (when they must).
http://blogs.discovermagazine.com/science-sushi/2015/06/30/flatworm-hypodermic-self-insemination-by-injecting-sperm-into-their-own-heads/#.VZRNuRtVhBe
http://blogs.discovermagazine.com/science-sushi/2015/06/30/flatworm-hypodermic-self-insemination-by-injecting-sperm-into-their-own-heads/#.VZRNuRtVhBe