https://www.digitaljournal.com/tech-science/amazing-mating-behavior-of-the-flatworm/article/437336
Injecting sperm into the head, how flatworms might mate - Digital Journal
Jul 2, 2015 - Biologists have been studying the mating behavior of a species of flatworm called Macrostomum hystrix. If the animal cannot find a mate it will subject
the headmate digitalinjectingsperm