Flatworm Penis Fighting

Flatworms are organisms that are part of the phylum Platyhelminthes. Flatworms are soft-bodied sea creatures that don't have segments and are bilaterally symmetrical. They are more advanced than Cnidarians due to their digestive tract, but less advanced than Molluscs because flatworms only have a one-way gastric passage.

One really cool thing about flatworms are their penis fights. They are hermaphrodites, meaning each one produces both sperm and egg. When two flatworms find each other, they start a "penis fencing" duel. Each worm has to try and stab the other with its penis and the worm that is inseminated becomes the mother. Alternatively, both worms stab each other with their penises and inseminate one another. This process is known as bilateral sperm transfer and results in both worms giving birth. When the sperm exits the male's penis, it enters the haemocoel of the other worm. The haemocoel is a blood filled cavity in platyhelminths that works as their open circulatory system.

Sources:
https://en.wikipedia.org/wiki/Flatworm
https://en.wikipedia.org/wiki/Penis_fencing
https://upload.wikimedia.org/wikipedia/commons/2/2b/Mating_Pseudobiceros_bedfordi.png