The flatworm has long been one of our favorite creepy crawlies for the very reason demonstrated in the video below:

They have both boy parts and girl parts, and they fence with their penises to determine who will be the male and who will be the female. What the hell could this possibly look like?

Check it out in all its feakiness below: 

Imagine if every time you wanted to have sex, you had to fence with your junk to decide which gender you'd be....how totally twisted.