Flatworms are described by Wikipedia as
“a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals…have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.”
Now… Here it sounds like the flatworm kind of got a raw deal in the course of evolution. A “simple” creature, so simple in fact that it doesn’t even have a body cavity or respitory organs! What a loser! And a lazy one at that: The majority of species of flatworm are parasites and so rely on other organisms to live! And just to point out another flaw while I’m at it, some of them don’t even have an anus and they regurgitate their waste back out the only opening they have: Their mouth. EEWWW. Jeez, I’m glad I’m not a flatworm! But hold on, there are THOUSANDS of species of flatworm, and it just so happens that some of them are particularly interesting. If you are a creep like me for whom “interesting” directly translates as “does cool sex stuff”
This is a turbellarian.
Turbellaria are a species of flatworm that are described as mostly free-living. Now, the image I get in my head when I think of free-living is this:
But in biological terms, this simply means that an organism is not parasitic on another individual. In other words, being a “free living” flatworm means that you move in order to eat. Those crazy free-living worm-hippies.
All turbellaria are hermaphrodites, meaning that they contain both male and female reproductive sex cells, and can reproduce both sexually (as in when a lady and a man love each other very much..) and asexually (you split yourself in two to make an ickle clone of yourself. No other individual involved. Effective but presumably not as much fun). An interesting question lies here. If an organism can produce asexually, why not just do that all the time? After all, sexual reproduction is harder work and a larger drain on our resources than we might think. Firstly you have to find a mate. And then they have to mate with you. And then you end up with offspring that is only made up of about 50% of your genetic material. But, if you produce asexually (i.e. on your own), you end up with a clone of yourself which is made up of 100% of your genes and what could be better than that?!
Unfortunately, multicellular organisms who rely completely on asexual reproduction tend not to last very long, because if you are only ever making clones of yourself with none of the genetic variation that come as standard with gamete-mingling sexual reproduction, you are leaving your offspring incredibly vulnerable to disease and ultimately extinction. So although asexual reproduction is good for some reasons under certain circumstances, it is useful to switch between sexual and asexual strategies. And that is what the turbellaria do. And when they do engage in sexual reproduction, penis fencing is often involved.
THAT’S RIGHT. I SAID PENIS FENCING.
Fencing. With a penis.
Or four penises (peni?) as the case may well be with this species.
Those two little white nubbins you can see at the top of each organism there? Yup. Those are penises. And what these flatworms apparently do is try to fertilise the other. Ultimately, the “winner” is the one who manages to pierce the skin of the other, and insert their sperm. The one who loses out in this unfortunate battle is the one who becomes the “mother”, as they have to invest much energy into producing the offspring, while the “father” simply deposits his sperm and leaves. Who says romance is dead?
The below video is probably NSFW, especially with the sound on. You’ll see what I mean. Bom-chikka-wow-wow! FLATWORM PORN!