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Thank you to Skillshare for supporting this episode of Journey to the Microcosmos.

And if you are one of the first thousand people to click the link in the description you can get a two-month free trial of Skillshare’s Premium Membership. There are no ghosts in the microcosmos.

There is no Halloween in the microcosmos. But if there were, this flatworm would fit in quite nicely with its spectral appearance. It might be navigating its way through a field of other organisms, but it looks like it had three minutes to prepare for a costume party and decided to go with the classic “old bedsheet with eyeholes” ghost costume.

And maybe comparing this flatworm (or even worms in general) to a ghost is kind of apt. Ghosts straddle two worlds, and so do worms, though in their case, instead of bridging the realm of the living and dead, worms manage to stretch between the micro and the macro. This creature you’re watching now would probably be difficult to see without a microscope.

But on the other end, there are worms that are meters too long to observe with a microscope. And akin to ghosts, worms might seem like a sort of reduced form of life. After all, for a lot of us, worms are the wiggly tubes that we dug up from playgrounds.

And of all of the worms, flatworms are perhaps the most simple of all. But even digging into what it means to be the simplest worm makes the whole notion of simplicity seem ludicrous. So let’s start with the broader question: what are worms, other than wiggly, self-mating tubes?

While we generally use the word “worm” to describe many different, unrelated, invertebrate, tubular animals that belong to a number of different phyla, there are three main phyla that people focus on. The first are our flatworm buddies here, who are known more formally as Platyhelminthes. Then there is the phylum Nematoda, known less formally as roundworms.

We’ve come across nematodes many times in our journey through the microcosmos, which makes sense given that they are one of the most abundant animals on this planet. And last are the Annelida, phylum of our playground friend, the earthworm. While earthworms are a bit beyond the scope of the micro, if you will, we’ve also got this little Stylaria lacustris.

At the tip of its head is the proboscis, an antenna-like structure that the worm uses for feeding. And as we scan past the head, you can see that the length of the Stylaria’s body is segmented. Of the three phyla we’ve mentioned, annelids are the most complex.

They have a closed circulatory system with tubes that transport nutrients and oxygen throughout their body, as well as a complete digestive system that has both a mouth to take in food and an anus to get rid of waste. If we think of worms in general as a gut enclosed within a body wall, then that gut is a tube and the body is a tube, and you can imagine that worms are a tube inside a tube. Now what separates these different groups is the space that separates one of those tubes from the other.

Complex systems inside annelids are due to what’s called a coelom, a tissue-lined cavity that sits between the tube of the digestive tract and the tube of the body wall. Importantly, this cavity is filled with fluid, which facilitates the development of organs along with the transport of nutrients around the body. The development of the first coelom was a very big deal because coeloms don’t just allow annelids to have complex systems.

They connect the seemingly simple organisms we’re talking about here to more complex animals—like us! Because annelids and humans are both what is known as coelomates, or eucoelomates. We are animals that have coeloms.

So, thank you to the ancestor that we share with earthworms who developed the first coelom, who allowed more complex organisms like you to exist. Nematodes, on the other hand, are what are called pseudocoelomates. They still have a coelom-like fluid-filled cavity, but the cavity isn’t lined with tissue like you find in true coeloms.

Other microscopic pseudocoelomates include gastrotrichs and rotifers. This pseudocoelom is a slight but significant difference in nematodes when compared to annelids, corresponding to a slightly less complex body plan, though nematodes still do have a complete digestive system. The flatworms are the simplest of all.

Starting with the coelom: flatworms don’t actually have one. They’re considered acoelomates. That area between the gut and body wall where annelids and nematodes have a cavity full of fluid is instead full of tissue.

This has a few consequences for the flatworm, but one of the biggest ones is that this area can’t support the development of specialized systems that you see in other animals. The lack of coelom imposes limits on the flatworm’s body and lifestyle, reducing both its complexity and size. But that does not mean the flatworm lives a simple life.

Of the three worm groups, flatworms are the most like a tube within another tube, except that their outer tube is much less cylindrical. The “flat” in “flatworm” is actually an adaptation: without a circulatory system or respiratory system, the worm relies on diffusion across its outer membrane to supply cells with oxygen. Being flatter gets the cells closer to the outside and better positioned for diffusion.

And without the ability to build a complete digestive system like those found in nematodes and annelids, flatworms have to adjust to life without that most under-appreciated body part of all: the anus. They take in food through one opening, digest it in their gut, and then spew the waste back out from that same opening. So unlike other organisms who have separate entrances and exits for their food, the flatworm must wait to finish digesting before it can eat again.

But this is not the only way flatworms can get rid of waste. Their body is lined with a special type of cell called a flame cell that also gets waste out of the worm. While it may not have the elegance of an interconnected set of organs, this system is its own form of complexity.

Basically, every flame cell is one single-celled kidney, allowing the worm to just...ooze waste from its skin. And this is just scratching the surface of the biological intricacy flatworms are capable of. Like many of their more involved worm counterparts, flatworms can reproduce both asexually and sexually.

And when reproducing sexually, they can mate with another flatworm, or even with themselves--both making and fertilizing their own eggs. There’s also those flatworm species that can regenerate, a trick that our more developed bodies are, alas, not capable of. Even the many weird ways they move make the word “wiggle” feel reductive.

Our perception of simplicity across nature is built on comparisons. Compared to us, an earthworm may seem simple. Put that earthworm next to a flatworm, and suddenly the earthworm represents a gigantic biological advancement.

But then put that flatworm next to a bacteria, and suddenly it contains multitudes, it is a universe. That flatworm is a marvel, an almost unbelievable testament to the power of evolution. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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