|Image: Richard Ling|
Proof at last that biology isn't all "deoxyribonucleic acid" and "atrichous izorhiza"!
They belong to a phylum called Platyhelminthes.
That didn't last long...
Platyhelmenthines is divided into 4 classes, three of which are entirely populated by such parasitic superstars as tapeworms and flukes. All the flatworms that aren't parasites belong to the fourth class, Turbellaria.
|Image: Mateos, Eduardo|
A dopey planarian
Altogether there are about 4,500 species of flatworm in Turbellaria. They're divided into about a dozen orders distinguished by fine, intimate details regarding their reproductive organs, gut and pharynx. Most are free-living but an entire order of strange, finger-faced Turbellarians is completely full of flatworms who live either within a host as a parasite, or on a host as a lazy, mostly harmless lodger.
Poor, old flatworm. It's just so hard to not be a parasite when you're so good at being a parasite!
|Image: Eduard Solà|
Land planarian with adorable eyes!
Video: JohnJohanna Wilson
Waving their cilia like this creates little whirlpools about their body. One might say the water is perturbed. It's turbulent. Or you could just say that "Turbellaria" comes from the Latin for crowd or commotion.
When the aquatic ones want to get around more quickly they wriggle their entire body like a snake or eel. Or bed linen in a breeze. They can't all do that, though, because despite the wonderful simplicity of the name "flatworm", not all flatworms are flat!
The thing about flatworms, the real thing with them is, they don't really bother too much with organs. They have no circulatory or respiratory systems. No heart or lungs. No blood vessels.
Flatworms are different. When their cells need oxygen, they absorb it from the outside world.
This cuts out an entire army of middlemen, but it does mean that all of their cells need to be close to the outside world. For larger flatworms this means being extremely flat. Really tiny ones, however, can get away with being oval or quite round. Unfortunately "roundworm" is already taken.
One thing flatworms do have is a gut. They don't have a coelem but they do have a gut. Oh, a coelem is a body cavity, a space between the outer body and the organs within. Even earthworms have one, which makes them less like one long tube and more like a tube within a tube. Roundworms make a noble effort with their pseudocoel. Flatworms are just solid fleshiness all the way through.
As for the gut... we usually say food goes in one end and out the other. First of all, their mouth doesn't have to be on one end, it can be right in the middle. A Turbellarian's entrance can be anywhere on their underside.
|Image: Scott Beardsley|
A planarian with its gut full of black ink
When it comes to anus, Turbellarians are real blue-sky thinkers. They're willing to think the unthinkable when it comes to designing an efficient, cost-effective waste disposal system. I bet it came in on time and on budget, too. We can learn so much from Turbellarian anus.
Most Turbellarians are scavengers or carnivores who hunt whatever creatures are small enough to fit in their mouth. They could be minute, single-celled organisms, the tiny larvae of aquatic insects or even oysters and mussels. Some Turbellarians have their mouth at the end of a muscular tube which they plunge into their prey to suck out the innards.
|Image: Aaron Muderick|
Turbellarians are blessed with a simple brain and most have a pair of simple eyes that can discern which direction light is coming from. Usually they'll go in the opposite direction. Some have 6 eyes and there are even a few large species with eyes all around the edge of their body. They also have cells for touch and smell, which is probably the main way they find food.
|Image: Richard Ling|
Orange bit are the pseudotentacles
And then there's reproduction. Goodness me.
Turbellarians are simultaneous hermaphrodites, meaning they have both male and female sexual organs. They meet up and do a simple, like for like trade. Eggs develop and later hatch into miniature adults.
Some species are little different; they don't want to be left holding the baby. When they meet, they rear up, pull out a pair of sharp penises and attempt to stab each other with sperm. They don't have to aim anywhere in particular, just stabby stabby stab stab and the sperm goes where it needs to go. Hypodermic insemination achieved.
The loser is blighted with motherhood while the winner successfully demonstrates who's too pretty to survive prison.
This isn't the only violence that can give rise to a new generation of flatworms. Many can reproduce by fission, where one animal splits in two and each part develops into a new adult.
|Image: Christopher Laumer|
A chain of catenulids
But the real masters exhibit near supernatural powers of regeneration. If you decapitate one, the head will crawl away and grow a new body. Meanwhile the body will crawl away and grow a new head with a new brain inside. You don't have to be careful with your decapitation, either. For the very best regenerators a single, inch-long individual can be sliced and diced into hundreds of pieces and each one will develop into a whole new adult.
Research has shown that brand new brains can retain old memories! Planarians can be trained so they no longer fear light and can placidly feed without running for shelter. If you cut off its head, the headless body will not only develop a new head, but the new brain can also be "reminded" of the training its predecessor had. With a fraction of the effort it took for the original planarian, this new brain can quickly learn not to fear light.
It's clear: the true nightmare is not the zombie apocalypse. It's Attack of the Giant Flatworms - When Bed Linen Kills. Imagine smashing a man-eating flatworm into smithereens and each smithereen grows into a new, man-eating flatworm! The horror...