Flatworm

[Shiny180]

Very interesting when flatworms mate, two 'males' use their bifurcated penises to fence one another. The winner, or the flatworm who stabs the other with it's penis, remains make while the loser becomes female.
Flatworm Mating: Literally a Cockblock | I ****ing Love Science

 

[chippie2002]

Seriously? Does the man do a runner afterwards?