Turbellarian Flatworm

Image: Richard Ling

Flatworms! They're flat worms. So they're Flatworms!

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...

Image: prilfish

But at least Platyhelminthes comes from the Greek for... "flat worm". There are tens of thousands of flatworms in the world, and maybe even a few in your intestines RIGHT NOW, because the vast majority of them are parasites. Flatworms are really good at being parasites.

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

Tubellaria contains all those colourful, ruffled jelly-carpets one might find in coral reefs, the cross-eyed planarians often found in lakes and our very own Land Planarians. Not to mention a whole host of other species ranging from 1 millimetre long things which glide between grains of sand, to 60 cm (2 ft) beasts which rummage through leaf litter.

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!

Tubellarians don't have much defence against water loss so terrestrial ones have to remain in dank habitats in the undergrowth while others live in the sea or in freshwater. Each one of their skin cells is armed with several hair-like cilia which they wave around so they can creep along on a bed of their own, secreted mucus.

Video: Howies Lab

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.

Video: WeirdunderWaterWorld

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.

Image: Gieysztoria

So imagine a cell gasping for oxygen. What do we do? We have to take it in from the outside world, so we breathe in a lungful of air so that blood can absorb the oxygen before the heart pumps it through an intricate network of blood vessels that ensures every cell can absorb the oxygen it needs.

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.

Video: microuruguay

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

Secondly, it doesn't necessarily go out the other. Some Turbellarians have a gut like a simple sack, so they have to vomit out the waste products. Some of the longer ones have an anus on the other end so they don't have to bring the waste all the way back up their body. Some have a branching gut because again, with no blood vessels, the actual gut has to spread out to make sure every cell can absorb food. Some Turbellarians even have more than one anus, because bringing up waste from all those branches would be difficult.

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.

Video: mathxp

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
Yuck

Some terrestrial ones are a lot more disgusting. They smother their prey, secrete enzymes that dissolve the body of their victim and have a grand old time wallowing in the resultant slurry.

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

Some of the colourful, marine flatworms have a pair of tentacles-but-not-really-tentacles on their head. They seem to stick out like tentacles should, but they're just a bit of the flatworm's flat body folded up a little. It's like if you saw a hooded figure on a dark and stormy night but when you look closely, you realise there's no actual person in the hood. And the hood is your own bed linen.

*spooky*

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.

Video: NatGeoWild

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

One order within Turbellaria is called Catenulida, meaning "small chain", because they can end up as a whole conga line of flatworms all connected together.

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...