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Butts. A Whole Compilation Of Them.

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Hi, I’m Sam Schultz.

I’m new here on Journey to the Microcosmos and I’ve been put up to a very questionable task for my first episode. While the other new hosts get to talk about beautiful microbes like the Beggiatoa, I am here to talk to you about butts.

A whole compilation of them. And I know why I’ve been asked to do this. I’m a producer for SciShow and one of the hosts for the lightly competitive science game show podcast SciShow Tangents.

And each of our episodes ends with a butt fact, where one of our panelists recites a fact about butts or poop or something vaguely butt-related. We’ve also been known to argue about important existential questions that can shake you to your very core, like…is butt legs? But as someone who is on Team “Butt is Legs,” I have to admit that microbes make a compelling argument for the opposition, because so many of them don’t have legs but still have a butt.

Like in this first video, about the flowery-looking animal called Bryozoa. When you first see them, you’ll probably be like, “Wait, where’s the butt?” But stick around, and you’ll find out why that’s actually a very important question for scientists studying the Bryozoa. If I were to ask you to just do a little sketch of what you think of when you hear the phrase “moss animals,” you might imagine something like this tardigrade—a waddling, slightly fantastical creature that we did in fact find living within the minuscule greenery of moss.

But depending on who you are and what you know, maybe you draw something completely different. Maybe you draw something more like this. These don’t really look much like an animal at all, do they?

At first glance, they seem a bit more like plants, with a stalk connecting a series of flowers with thin, elegant petals. But no, that is indeed an animal. One that has the dubious honor of being defined largely by its anus.

Before we get there though, let’s start with just explaining what these animals actually are. These organisms are members of the phylum Bryozoa, a name that translates in Greek to “moss animals”. But Bryozoa aren’t actually animals that live in moss.

They get their name because of what they look like when all of the elegant individuals you see here congregate into colonies that look, ya know, mossy. That giant gelatinous mass seems to bear little resemblance to the tiny winsome creatures that make it up. But Bryozoa are notable in the animal kingdom because of just how colonial the phyla is.

Around the world, in waters salty and not, are colonies of different shapes and sizes made up of individuals like these. These colonies can get their start in different ways. Some might be the result of sexual reproduction, sperm and egg that could have met in the waters away from their makers.

Or, in other cases, one colony may have captured sperm released by another, using it to fertilize its own eggs, which stay brooding until a larva emerges, ready to set out and develop its own colony. But Bryozoa have another asexual option for reproduction. Instead of fertilizing an egg, a colony can amass a bunch of cells into a structure like this one, called a statoblast.

These statoblasts can be so prevalent in our samples that James, our master of microscopes, has to scoop them out to keep them from being the main focus of our videos. They are thick and durable, able to lay dormant and protect their contents from cold temperatures and dry weather. But when the conditions become right again, they grow into a new individual that will form the basis of a new colony.

To do that, the individual Bryozoa, called a zooid, navigates the waters as a larva, eventually attaching itself to some kind of solid surface. Perhaps a piece of rock, or some kind of shelf, maybe even some seaweed. And from there, it will begin to divide, budding off but keeping its descendants close and connected as they increase in number.

Now, the arrangement of these zooids dictates the shape of their colony, as does the material they secrete to support the colony overall. Some excrete a gelatinous material that turns them into those masses of jelly we saw earlier. Others rely on chitinous materials that give them more rigidity.

And as they spread over their surface and even grow up into the water around them, Bryozoa colonies take on many spectacular shapes. Even though Bryozoa are different from coral, they definitely look similar. And like coral, Bryozoa colonies don’t move.

Instead they remain attached to their surface. But while they may not be able to move through the world around them to gather resources, Bryozoa are able to make the most of their surroundings, with individual zooids taking on special functions that allow the group to accomplish more than the individuals. Some focus on making eggs, while others form small beaks that allow them to snap at predators.

And these zooids are all dependent on another group of zooids that gather food and share the nutrients with everyone else. To get that food, the feeding zooids rely on the lophophore, the ring of tentacles crowning every zooid’s bodies that looks a bit like a bunch of thin petals or a very elegant duster. Those tentacles are covered in cilia, which circulate the waters around the Bryozoa, stirring currents like the ones you see here so that it can sweep bits of food like plankton towards the animal’s mouth.

And from certain angles, you can see how the lophophore and the rest of the zooid’s organs tuck into a sort of casing, which is a structure called the cystid. And while the Bryozoa has various other parts that we could talk about, what we’re really here for is the anus, which is a much bigger deal than you might expect from an animal with such a simple digestive system. There’s a mouth, an esophagus, and then a u-shaped gut that leads to the anus.

Now the issue is really rooted in the existential crisis at the heart of microcosmos: how are things related to each other? And what does that mean for the names that we give to things? Organisms like these were originally given the name Polyzoa in 1830, but that was changed a year later to Bryozoa by the German naturalist Christian Ehrenberg.

But as more organisms were added to the group, the people watching them and describing them realized that they were actually lumping together two very similar groups of organisms that actually had some key differences between them. And one of those key differences was where the organism’s anus was located. One group had their anus situated just outside their lophophore—that tentacle crown we were watching gather food earlier.

And one had their anus situated just inside their lophophore. So to differentiate between the two groups, scientists gave them very appropriate names. One group was called Ectoprocta, which translates in Ancient Greek to “outside anus”.

And the other were called Entoprocta, which means “inside anus.” Because you can define anything by its butt, as long as you turn to ancient languages. In general, Bryozoa typically refers to the Ectoprocts, though for many reasons (.. I can’t possibly imagine why…), Bryozoa seems to be the preferred name.

But the history and relationship between the Ectoprocts and Entoprocts remains an ongoing mystery that scientists continue to explore. They are not the only creature out there to be wrapped up in a phylogenetic mystery. But isn’t it great to know, especially for such a strange animal like this, that the mystery is built on butts?

Imagine if your name translated to “inside anus” or “outside anus”? I guess it sounds better in Ancient Greek, but still, scientists really make a lot of strange choices. Like the subject of our next video: Antoni Von Leeuwenhoek, the original master of microscopes.

Leeuwenhoek looked for microbes in all sorts of interesting places. But given the subject of our video today, I think you can figure out one of the more unique sources of microbial delights that Leeuwenhoek found. In the year 1675, I discovered very small living creatures in rain water, which had stood but few days in a new earthen pot glazed blue within.

Those are the famous words that Antony Van Leeuwenhoek used to describe microbes. Though he didn’t use the word “microbe”, he called them animalcules, and the letter itself is not exactly the one he wrote. Leeuwenhoek’s version was written, of course, in Dutch and then sent off to Henry Oldenburg, the secretary of the Royal Society in London who founded the publication Philosophical Transactions.

Oldenberg translated Leeuwenhoek’s work into English and published it in 1677. The rest, as they say, is microbiology. In 1932, the scientist Clifford Dobell published a biography of Leeuwenhoek called “Antony van Leeuwenhoek and his “Little animals,”.

Dobell introduces his subject with some self-consciousness for taking on the story of someone who, if you’ve been interested in microscopy, you have almost certainly heard of. And he wrote: Dear reader: I know full well that you and everyone else must have met Mr van Leeuwenhoek many a time before; but please let me reintroduce him to you, for he is a man worth knowing more intimately. Though he was born exactly 300 years ago he is still very much alive, and would be glad to make your better acquaintance—provided only that you are a “true lover of learning” (as of course you are).

The fascination with Leeunwenhoek’s work for Dobell and so many of us lies in the scope of Leeuwenhoek’s curiosity, and the writing that documented his many journeys into the microcosmos. Born in the Netherlands in 1632, Leeuwenhoek was a draper by trade, meaning that he sold cloth. He took up microscopy as a hobby, learning to grind lenses and turn them into a tool to see into tiny worlds.

The simple microscopes he built were better than many of those of his contemporaries, producing images that were both more magnified and more clear. Those of course compared to what the average high school biology class has today, they were very bad at both of those things. The creatures Leeuwenhoek described in his 1677 publication were not his first description of tiny animals in water though.

In 1674, he wrote of “green streaks, spirally wound,” a description that later observers have taken to refer to the filamentous green algae spirogyra. He also wrote of little creatures he’d found that were, as he described them, “a thousand times smaller than the smallest ones I have ever yet seen upon the rind of cheese.” Those little cheese rind creatures he was describing were likely mites. Reading Leeuwenhoek’s work is kind of like following an old treasure map.

His words lay out pathways and clues that help us navigate through a drop of 17th century water. And if you follow that map, at the end you find a wealth of information, the knowledge of microbes and their identities that was still unknown in his time, but that in the centuries since has been accumulated. For Dobell, Leeuwenhoek’s 20th century scientist biographer, this treasure hunt was a long one, borne out of his own enchantment with these original discoveries and a desire to better understand them.

He learned Dutch, only to find that the 17th century Dutch of Leeuwenhoek’s letters was impenetrable to him And with World War I raging around him, he had to set aside his attempts to translate Leeuwenhoek’s work for some time. But eventually, through what seems to be largely a lot of determination, Dobell was able to work his way through Leeuwenhoek’s letters. And that labor of love led, naturally, to translating things like Leeuwenhoek’s descriptions of his own poop.

Per Dobell, Leeuwenhoek wrote the following: I have usually of a morning a well-formed stool; but hitherto I have had sometimes a looseness of the bowels. […] My excrement being so thin, I was at divers times constrained to examine it. But Leeuwenhoek wasn’t documenting his stool just for the sake of it. In that sample, he saw an opportunity to observe the animals within.

I have at times seen very prettily moving animalcules […] Their bodies were somewhat longer than broad, and their belly, which was flattened, provided with several feet, with which they made such a movement through the clear medium and the globules that we might fancy we saw a pissabed running up against a wall. That description sounds promising, but Dobell had a problem. Leeuwenhoek had compared his microbes to a “pissabed,” and like us, Dobell had no idea what a “pissabed” is.

Even the original English translation he’d read shrugged it off as a Dutch term with no obvious English equivalent. Dobell had to find a 17th century Dutch-English dictionary to figure out that a “pissabed” is a woodlouse. And with that reference point, Dobell had enough to diagnose Leeuwenhoek—very belatedly, of course—with Giardia, the product of a flagellate known as Giardia intestinalis.

At the time that Leeuwenhoek was identifying the microbial residents of his stool, he almost certainly didn’t know their role in his poor digestion. Nor did he of course have any name for them, or any of the other microbes that we now know he saw. And yet we can identify rotifers from his characterization of their wheeled heads, and vorticella from his description of their belled bodies.

The irony of our centuries-later fascination with reconstructing and replicating Leeuwenhoek’s work is that his contemporaries had a much harder time of it. Leeuwenhoek didn’t help his cause much, refusing to provide extensive details of his methods and even keeping his microscope-building techniques to himself. He did, however, provide eight signed testimonies from various men, including a minister, to verify that he had made these observations.

Of course, signed testimony is one thing. Reproducibility is another. Several of Leeuwenhoek’s contemporaries tried and failed to replicate his observations until finally Robert Hooke—the famed microscopist and writer-slash-illustrator of Micrographia—was able to see the tiny animalcules Leeuwenhoek had seen.

Hooke however was willing to outline his specific methods and demonstrate them, validating the observations Leeuwenhoek had made and providing others with the means to make their own microbial ventures. And like Dobell centuries later, Hooke was reportedly so taken with Leeuwenhoek’s work that he learned Dutch to read his letters. The legacy of Leeuwenhoek is in the microbes he uncovered, but of course it was more than that.

It was also the curiosity that he passed on, and the acts of interpretation that they inspired—both literally with all of these scientists taking up Dutch to just learn from his letters, and then scientifically as we decipher what microbes he must have seen. And so it seems fitting to close with how Leeuwenhoek viewed his own little animalcules. He said, “Among all the marvels that I have discovered in nature, these are the most marvelous of all.” I know I’m supposed to think it’s gross that Leeuwenhoek dug into his own poop for microbes.

But as weird as it is, I also respect the dedication to his craft. But now I want to know how many other noted scientists have documented and studied their poop so thoroughly that future historians can diagnose them with an intestine parasite. Is it just Leeuwenhoek, making him a true legend?

Well, I’ve probably spent too much time already thinking about Leeuwenhoek’s butt. Which brings me to my next question. What’s the big deal about butts anyway?

Why do we really need them? Luckily there’s an animal that can teach us that: flatworms, a worm so simple that it doesn’t have an anus. 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 bed sheet 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 are 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. I never really thought about how hard it would be to eat if I had to poop out my mouth at the same time, but yeah, I guess I have a newfound appreciation for the fact that our bodies are equipped with an easy exit strategy for waste.

But you know who I wish would poop a little less? The subject of our next video. Because now I can’t help but look around my house and wonder how much of it is covered in their poop.

One of the most frequent questions we get asked on Journey to the Microcosmos is how James, our master of microscopes, stays safe while gathering and studying samples. After all, as beautiful as the microcosmos is, there are plenty of parasites and pathogens in there that we’d prefer to keep under the microscope and out of our bodies. But while James has simple and effective guidelines to protect his health—like making sure he always washes his hands before and after handling samples, and never venturing into a pond with an open wound—there are times when those measures don’t quite cut it.

These are the times when no matter how much you protect yourself outside or inside, the microcosmos gets the best of you. These are the times of the dust mite. Now we don’t usually start these videos with a warning, but we’re going to do that today.

While working on this video, at least 4 members of the Journey to the Microcosmos team found themselves having a bit of a crisis about how clean their homes were. If you’ve found yourself sneezing and wheezing and running around miserable with a runny nose because of dust, this arachnid is probably the culprit—though perhaps not in the way you would expect. Dust mites are small in the grand scheme of things, but large in the microcosmos, hitting just around 300 micrometers in length.

Their bodies are decked out with four pairs of legs ending in feet that act like suction cups as they wander in their quest for three things: water, food, and darkness. Dust mites love humidity because their lives depend on it. They don’t drink water so much as they osmosis it into them, relying on a salt-filled gland near their mouth to absorb water vapor from the air around them.

So if their surroundings become more arid, an adult dust mite will wither away until it eventually dies. This is a challenge for dust mites that live in places with seasons, where wet and rainy falls give way to dry winters. To survive those shifts, they rely on the resilience of their younger brethren—the nymphs—whose forms are able to withstand the lack of moisture.

The seasons of the dust mite’s life was one of the important clues that helped scientists in the 20th century understand their role in dust allergies. Scientists identified dust as the source of some kind of allergen in the 1920s, but they were not sure what it was exactly about dust that did it. They just knew that people all over the globe suffered from dust allergies, and that these allergies were often seasonal, peaking in the autumn, particularly after warm, humid summers.

And other possible sources, like animal dander or mold, just didn’t quite fit right with the seasonality of the allergy. But in the 1960s, scientists in the Netherlands and Japan realized that the dust mite might be the culprit. They’re found in large numbers in the dust that lines the unwashed and untouched corners of our homes, their populations peaking in time with the runny noses and bleary eyes of dust allergies.

But while experiments confirmed that mites really were the source of people’s sensitivities to house dust, it wasn’t clear what made them so special. So, we’ll get to that in a bit... James often finds dust mites in the humidity chambers he uses to keep his microbes alive.

These chambers are made up of a dish lined with wet toilet paper, which is just about all a dust mite needs to feel right at home—especially when there’s potentially some delicious microbes to munch on. So to keep those other microbes from becoming a mite meal, James has to regularly clean out and disinfect the humidity chamber. Now, that cleaning might get rid of the dust mites in the humidity chamber, but the problem is that our houses are full of food for them because our houses are full of us—of flakes from our skin that shed and gather all around and sustain the invisible mite.

And this is a problem not just because it means the mite can thrive in our homes. It’s a problem because food means poop. Over the course of its life, a mite will produce about 1000 pieces of poop that are roughly the size of a grain of pollen.

Inside those bits of fecal matter are enzymes that help the dust mite eat its own poop and get nutrients that it might not have gotten the first time around. But should the dust mite choose to not revisit its prior meals, the feces will float around the room, attached to other particles until eventually they settle down—perhaps on a pillow, or on a pet’s bed, or a car seat. It’s like we’re living in a gigantic snow globe, except that the snow... is dust mite feces.

In 1981, researchers confirmed the bad news about this animal’s poop— people are allergic to it. To be more specific, they confirmed that dust mite poop contains specific proteins that many people are allergic to. So if you’re airing out some sheets and you start sneezing, what you might actually be reacting to is dust mite feces flying around the air.

Now, we apologize for this mental image but it is reality, and it has embedded in our heads, so we have to share it with you as well. But we can offer what might be a small comfort: there is another allergen from dust mites that has nothing to do with poop. Dust mites have a fairly lengthy mating process, sometimes taking up to two days to finish—this is a pretty lengthy time for any organism, but especially for an animal that’s only got about 100 total days to live.

Over the course of their life, the female dust mite will lay up to 80 eggs, which hatch into larvae that then go through several different stages of development before becoming adults With each passing stage, the dust mite sheds its exoskeleton, leaving behind its youth. And that exoskeleton provides some of the other allergens for people react to. But though that is something people are sometimes allergic to, it really is mostly just the poop that sets off people’s allergies.

So I guess there really wasn’t that much comfort there after all. And even for a trained and cautious microbiologist, dust mites can become an unwelcome surprise. Once, James brought home some samples taken from a water dish that his neighbor left outside for their cat.

The surface was covered in tiny round things, and James thought they might be rotifers. But when he looked at the surface of the scum under the microscope, there were no charming rotifers to be found. It was mites, just hundreds of them crawling around the slide.

It was so unpleasant that James immediately bleached the slide. Now, he’s not sure if that sample is the reason, but for days after, James kept sneezing and having to deal with a runny nose, all of which pointed to a potential mite invasion. Fortunately though, our homes are not a utopia for dust mites.

What they really need is dark, which is why they prefer to dig themselves deep into carpets and other soft things that give them space to burrow4. Dust mites have a harder time with materials like suede that are difficult to hide themselves in, and they usually avoid hard surfaces that are exposed to light. So for James, his weapons against the mite invasion were clear: a vacuum, a mattress cleaner, and a strong UV light bulb.

It is one of the more ignoble ends to one of our organisms. We don’t like to hurt them, unless they are hurting us. But I am sure there’s a dust mite somewhere in our homes right now, settling into a soft, dark abode of its own, with hardly a care in the world for the battle that was waged to bring you this video.

I don’t know what’s grosser. The idea of eating and pooping out of the same hole, or the fact that I’m constantly inhaling dust mite poop. So let’s end on a happier note, a festival of feces, if you will, unfolding in the diverse strategies used by both single-celled and multicellular organisms to get rid of their waste.

Ah, food, one of life’s great pleasures. Whether you’re an organism whose body can create its own food, or you make do with the consumption of other organisms, food sustains and connects life both large and small. But we’re not going to talk about food today.

We’re going to talk about what happens…after. No great meal comes without consequences. Even photosynthetic microbes produce oxygen they don’t need, a byproduct that is quite useful to many other organisms, including us.

And for organisms that have to digest their food to extract nutrients, well, there’s all sorts of stuff left behind when the process is done. In the world of the microcosmos, there are not only many varied tiny organisms. There are also many varied ways to produce even tinier poop.

And when we say “tinier,” we mean it. If you’re worried about being grossed out by this video, or you really want to make sure you’re not missing anything, just know that some of this pooping will require you to pay very close attention. Some of this will probably look similar to macroscopic bowel movements, and some will seem less so.

So let’s start with the more poop-like poop that comes from some of the multicellular members of the microcosmos, particularly the ones with digestive systems complete with specialized regions for digesting food and releasing solid waste. Our dragon-like friend the gastrotrich is doing the old scrunch-and-poop, probably after having finished a nice meal made up of bacteria or protozoa. The food came in through its pharynx and was then sent through the intestine, a straight tube full of digestive cells.

After, the remains continued their way to the end of the tube, where lies the anus--a microscopic gateway out of the gastrotrich and into the world. That’s all well and good for the more complex organisms, but how does one poop when one does not have a butthole? Well, for some single-celled organisms, one becomes the butthole.

Such is the case for amoebas. Their formlessness allows them to easily shift their shape around to take in food via phagocytosis. The amoeba here happens to have found a nice meal made of a ciliate, which is now contained in a food vacuole filling with digestive enzymes.

Those enzymes will eventually break the ciliate down, allowing the amoeba to absorb nutrients into its cytoplasm. But then what happens with the remains? Well, as we see with this amoeba here, it gets released.

Amoebas and other similar organisms use a process called exocytosis to send waste back to the membrane where it is removed from their body. You can see this again here in this Mastigamoeba, slowly gliding along the microscope slide while a round bit seems to come off up top. It almost looks like it’s leaving behind a little piece of itself, which I mean, maybe it is.

Ciliates have a more refined approach to pooping, which is to say they have the unicellular equivalent of an anus called the cytoproct. Ciliates gather food into an oral groove, and then consume it in the digestive vacuoles that travel through the cell. Eventually, when the vacuole makes its way to the cytoproct, the contents get out, and there you have ciliate poop.

The way organisms poop—the way microbes poop—are even more varied and wonderful than we’ve been able to show you here. But you might be wondering how you even get from something relatively simple like this ciliate releasing a small bit of waste to something more complex, like this tardigrade with its more compartmentalized approach to defecation. Well the answer, of course, is evolution, but it’s an answer that leads to many more unanswered questions.

Even within animals, the evolution of the anus—or in some species, the cloaca—is not well understood, as discussed in a 2015 review paper with the spectacular title, “Getting to the bottom of anal evolution”. While the anus itself is linked to the evolution of a digestive system, not all metazoans have a designated anus. The hydra gut has only one opening, which means that the organism’s mouth has to pull digestive double duty and open back up to release the waste.

And how that system fits in with the evolution of digestion overall is unclear. Much like the evolution of sex, anuses are a trait that appears and disappears in evolutionary lineages, reflecting the fact that they might be useful for some animals and less so for others. The duality of the hydra’s mouth seems to be working out great for them after all.

But having a digestive system with a distinct entrance and exit is pretty handy too. For one, it’s just a little less disgusting. But it also keeps food flowing in one direction.

And that means animals like the tardigrade and us don’t have to wait to finish digesting to eat more food. That would be annoying. We just get to keep eating, all thanks to that evolutionary mystery that is the anus.

In the end though, anus or not, everybody poops. Food, after all, might be one of life’s great pleasures, but all good things must come to an end. And perhaps, with the microcosmos, we can find some pleasure in that as well.

And that is the conclusion of our journey through the hind end of the microcosmos. It was beautiful, it was gross. And I hope, most of all, that unlike the nutrients these organisms were getting rid of, this video was not a waste.

So, thank you for coming on this journey with us as we explore the unseen world that surrounds us. Before we go, we’d also like to say thank you to each and every one of our Patrons. Some of their names are on the screen right now, and these are the people that make this channel, and videos like this, possible and for that we are so grateful.

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