http://www.cracked.com/article/241_5-animals-that-can-do-amazing-things-...

"The cockfights (GET IT?!) are what keep the species going. Since flatworms are hermaphrodites, when two of them happen to meet they'll get erections, open their mouths and start stabbing until someone is either pregnant or dead. It's strangely mesmerizing. Seriously, we could watch these videos all day."