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You might think worms are boring and gross, and it's true that they're often a bit slimy. But they're anything but bland. Between true worms, flatworms, roundworms, and all the other things we lump together as worms, there are tens of thousands of different kinds of worms on this planet.

And they're unbelievably complex and fascinating. In fact, we can learn a lot about ourselves by studying them. So today, we're bringing together science stories about different sorts of worms, to reveal just how wormderful they are.

Let's start with the worms you're probably most familiar with: earthworms. They're a favorite of composters and fishing enthusiasts alike. And if you're looking to grab a few for either purpose, you'll probably have the best luck after a rainstorm. Here's Olivia, with what we know about why that is.

After a heavy rainstorm, you've probably seen worms popping out of the soil, getting stranded all over sidewalks and parking lots. But between birds, clumsy humans, and the sun, it's dangerous for a worm to climb out of its safe, cozy burrow. And yet they suddenly abandon ship whenever it starts raining.

Scientists have come up with plenty of possible reasons why rain might trigger this behavior, but they don't quite agree on all of them. The oldest hypothesis, and one you might've heard, is that earthworms surface to avoid drowning. That's because worms take in oxygen through their skin in a passive process called diffusion, where oxygen moves from the higher concentration outside their bodies, to the lower concentration inside.

When soil floods after a rainstorm, worms can usually still breathe if there's enough oxygen dissolved in the water. But, water is a lot denser than air, so diffusion is much slower. Like, up to a thousand times slower. Which means that for a worm, it could be a lot harder to breathe after a downpour, which could make them rush to the surface.

But some biologists have pointed out that plenty of worms can actually live for days submerged in water, although some of that may depend on the species.

One study published in 2008 found that two different species of worm use different amounts of oxygen. A common tropical worm uses relatively little, so it doesn't come up to the surface after it rains, but another worm, the Alabama Jumper, needs more oxygen, especially at night, and it does surface. So there may be worms who do need to surface to breathe, or at least prefer to, while others don't.

Other experts think that worms might surface for different reasons. They argue that worms flee their burrows because rain gives them great travel conditions. Earthworms move faster above ground, and they can crawl more quickly and safely when it's wet, because they're less likely to dry out. Some scientists have also proposed that worms take flooded soil as a cue to mate, possibly increasing their chances of finding a partner, but that seems to be limited to nightcrawlers and a few other species. 

One last hypothesis is that raindrops landing on the soil create vibrations similar to those made by one of the earthworms' biggest predators, the mole. It's definitely true that earthworms will book it to the nearest exit if they sense certain tremors. People actually exploit this by going worm grunting, where they catch worms to use as bait by driving stakes into the ground and vibrating them. But experiments suggest that worms are only sensitive to vibrations that mimic moles and that falling rain doesn't do the trick.

So we may not know for sure why earthworms grace us with their presence after a storm, but at least we know what to do if we want to go fishing.

- [Michael] Who knew worm behavior could be so complicated and mysterious? Any worm biologist, I guess. After all, there are thousands of species of worms on this planet, and many of them are so bizarre they don't even seem real. For instance, in the depths of the ocean, there are worms without mouths or anuses. Their gut has no entry or exit holes. I'm just gonna hand it over to Hank to explain that. 

Buttless Worms
- [Hank] There are plenty of creatures out there in the world with only one opening to handle the business of both taking in food and getting rid of the leftovers.

Jellyfish, for example, get along just fine with a mouth that is also an anus. But there's at least one animal out there that doesn't have a gut opening at all. The giant tube worm Riftia pachyptila lives over a kilometer deep in the ocean along ocean ridges where hydrothermal vents are common and spew boiling hot, chemical-laden water into the freezing cold deep sea. These chemicals include stuff like hydrogen sulfide, which isn't all that great for most animals, but these bizarre-looking worms are adapted to live in this hostile habitat without so much as a mouth to make things easier.

Riftia almost looks like a lipstick with their vivid red plumes and tube-like outer casing. Within, they have a structure called a trophosome, which is like a gut but with no way in or out. They were first discovered in 1977 when the submarine Alvin accidentally landed on a cluster of them while it was investigating hydrothermal vents near the Galapagos. Red blood gushed up around the sub. The researchers on board later discovered that the tube worm's plume has blood vessels full of hemoglobin.

In us mammals, hemoglobin is mainly responsible for transporting oxygen. In Riftia, the tube worm's plume acts like a gill. Its hemoglobin helps pull that hydrogen sulfide from the vent water and move it into the trophosome. Inside the trophosome are millions of symbiotic bacteria, accounting for up to half of the body weight of the worm. The bacteria are able to convert the toxic vent water chemicals into a food source for the worm through a process called chemosynthesis.

Much like plants use photosynthesis to produce food using sunlight, these bacteria are getting their food through chemical reactions that use hydrogen sulfide to produce energy, and the inside of a worm, it turns out, is a much better habitat for them than the open vents, or so the hypothesis goes. Not only have the bacteria turned this hostile environment

into an advantage, the worms have capitalized right along with them. And they don't need a mouth or anus; they just get fed by their bacterial partners, which produce enough food to keep everyone happy. It's like having your kitchen in your body.

Okay, so that's how the food gets in, but how does it get out? The waste produced from digesting this food can be transported back out via the worms' bloodstream. Theres no need for either a mouth or an anus. But this makes you want to ask another question -- If there's no way in or out, how do those bacteria get in there Researchers asked this question back in 2006 and found that it's weird. The bacteria enter through the tube worm's skin when it's still a larva, so basically, this is a bacterial infection.

Hydrothermal vents are an unpredictable place to call home. Thanks to the constant tectonic activity happening along ocean ridges, they may be there one day and gone the next. Once the vents stop venting, the tube worms die, because their bacteria's food source gets cut off. The distance between them can be several miles, which is a long swim when you're a little worm, so researchers aren't sure how tube worm larva get from place to place without a food source. Hypotheses range from whale falls to shipwrecks, which could supply enough of the chemicals the bacteria need to stay alive.

The discovery of these weird worms and their unique way of eating has led to researchers finding chemosynthetic communities and ecosystems around the world, from elsewhere in the ocean to Yellowstone National Park. Who knew a giant mouthless, buttless worm could completely redefine the way we thought about how life works on our planet?

- [Michael] Convincing bacteria to make your food is a pretty neat trick, deep sea worms, but I think planarians have you beat. They can do something even weirder. They can remember things after you cut off their heads.

Heads which, to be clear, they can grow back. Let me explain.

Flatworm Memories
The planarian is a saltwater and freshwater flatworm known primarily for its regenerative abilities. Cut off their tail, and they'll grow it back. Cut off them in half, and they'll become two whole flatworms, even though they have a distinctive head region with a brain in it. What's really amazing is that, even after losing its brain, a planarian can remember things from before it was beheaded.

Planarians are pretty simple animals, but they do have brains that control things like sensory reception, and this brain is considered a true brain, not just a bundle of neurons. It has two lobes, it controls nervous function throughout the body, and it has specialized regions. This means their brains probably look a lot how ours once did, and this makes them a great analog for studying brain evolution, as well as asking fundamental questions about things like memory.

It's hard to know what memories a planarian has, of course, but you can train them to perform new, unnatural behaviors and then see if they retain that memory under different conditions. Planarians have simple needs. They like raw meat, and they dislike light and change, which, I get it. They're willing to get over their light aversion for a tasty meal, but if other things change at the same time, they just kind of freak out.

Like, if you take them out of petri dishes with smooth glass and put them into textured petri dishes, they are too weirded out at first to go for a piece of liver in the middle with a bright light shining on it. But if you put the flatworms in textured dishes for a while beforehand, you can passively train them to be familiar with that environment. Then, when you add the spotlight and liver, they go for it quickly, providing researchers with enough evidence of successful training. And when scientists have done this training regimen, they found planarians can retain such memories for at least 14 days.

That raises the question, of course -- what happens when you cut off their heads? We know that, within a week, the flatworms grow new, usable heads they can eat with and everything, and they can make it that long without eating, so they're fine. But presumably, if you have a brain, that's where your memories are, so since the 1950s, scientists have been trying to figure out what happens when the flatworms lose and then regrow their brains. 

And, most strangely, they seem to remember previous training. In a 2013 study, for example, the trained flatworms approached the food faster, even though their training happened with their former heads. The difference between trained and untrained planarians was not as big as it was pre-decapitation, so the researchers concluded that this memory retention isn't quite 100%. But still, it's bizarre that they could remember the training at all.

As for how they retain those memories, well, we don't really know. One intriguing idea is that memories of certain habitual behaviors are partially transferred to neural tissue outside the brain. So somehow, there are memories stored in other neurons in the body, which is just so completely against everything we thought we understood about memories. I mean, we talk about muscle memory, but even that is thought to occur in the parts of the brain that control how you move your extremities, not in those extremities themselves.

A lot more research is needed to fully understand this phenomenon and to figure out if it's unique to planarians or true of all brainy creatures. Because if human memories also exist outside of our brains in some way, understanding how could lead to better treatments for certain brain injuries, memory loss, or dementias, and that would be some pretty awesome stuff to learn from a headless flatworm.

At this point, you just wanna thank worms for teaching us so much, you know what I mean? They probably wouldn't understand, though. And not all worms are so helpful. There are lots of parasitic worms out there, and they can cause some nasty diseases, like heartworm, which plagues our sweet, beloved dogs.

Have you ever wondered, though, why we humans don't worry about getting heartworms? Because we can actually get infected with them, they just don't do well once they're in us. Here's me again with why.

Human Heartworm
If you're a dog owner, chances are you've heard of heartworm disease, and if you aren't familiar with it, well, brace yourself for some serious nightmare fuel. It's basically exactly what it sounds like, an infestation of worms, collectively known as Dirofilaria, in a dog's heart. These parasitic nematodes are transmitted by mosquitoes, and different species are found all over the world. Most common in North America is Dirofilaria immitis.

The adult parasites live and reproduce in the dog's pulmonary arteries, eventually clogging the chambers in the right side of the heart. This leads to all kinds of nasty complications, like fatigue, coughing up blood, and, ultimately, heart failure. And usually, if your dog has enough worms to show symptoms, getting rid of them is really hard. That's why vets encourage pet owners to use preventative medications that decrease the risk of infection and lessen the spread of disease.

And it turns out that lowering the risk of transmission is good for dogs and people, because humans can be infected with heartworm, too. Now, before you freak out too much, heartworm infections in people don't cause symptoms nearly as bad as they do in dogs, and they're super rare. Fewer than 120 cases have been reported in the United States since 1941. 

The main difference is that our immune systems aren't as easily tricked by the worms. You see, when a mosquito bites an animal with a heartworm infection, they suck up microfilariae, the earliest larval stage of the worm. Those mature through their next larval stages inside the mosquito and then migrate to its proboscis, the stabby part it uses to suck up blood and, arguably, its least endearing feature.

When that mosquito goes for another blood meal, be it from a dog or human, the larvae bust out and get onto the skin. Then they, no joke, crawl around until they find a way in, like the tiny hole made by the mosquito. From there, they have to wiggle their way through the skin tissue to get into small blood vessels so they can travel around the bloodstream, eventually making their way to the pulmonary arteries and the lungs. Inside the body, they grow and mature for about six months until, finally, the mature worms reproduce and release their little microfilariae back into the bloodstream, starting the whole process over again.

Now, where dogs and humans differ, is that usually the larvae never make it into the bloodstream. This is probably because the larvae transmitted by the mosquito are in their third stage of development, or the L3 stage, and studies of human infections with related species of nematodes have shown that our immune system is really good at recognizing and mounting a response against parasites when they're at this stage.

In fact, this is probably why human heartworm infections are considered rare to begin with. It's not that we're rarely infected, it's just that we rarely stay infected long enough for anyone to notice.

Though sometimes, a rogue larva does find its way into a person's lungs, but even when this happens, the worm never gets a chance to grow and reproduce the way it would in dogs. The immune system always spots it and sends cells to destroy it. This destruction process forms nodules in the lung tissue, which is usually how we end up figuring out that someone had heartworm infection. These nodules, called coin lesions, are rarely harmful. They're mostly just annoying, because other, deadlier conditions also cause them, so doctors have to take them seriously. 

As for why our immune systems end up finding and killing those worms, probably has as much to do with bacteria as it does with the parasite itself. Filarial nematodes like Dirofilaria have an intimate partnership with bacteria called Wolbachia. The bacteria live inside the worm's cells, and it's a mutually beneficial relationship. The worms provide the bacteria with amino acids for growth, while the Wolbachia are essential for the development of the worm's larvae. 

And the bacteria also play an important role in the parasite's ability to infect a host animal. Proteins produced by Wolbachia cause the host's immune system to start fighting a bacterial infection, a type of immune response called a TH1 response, even though there isn't a bacterial infection going on. Now here's why this is important. TH1 responses are counterbalanced by immune responses called TH2 responses. They basically do opposite things. TH1 responses promote inflammation, while TH2 responses dampen it. 

So the presence of Wolbachia can shift the immune system toward more of a TH1 response, but the TH2 responses are what our immune systems use to attack worms. So by inducing the TH1 response, the bacteria seem to essentially distract the immune system, allowing the worms to sneak around and proliferate. Some scientists even think a strong TH1 response helps the larval worms grow and mature.

Also, dogs really get the short end of the stick here, because studies have shown that TH1 response is increased when the host has more microfilariae in their system. So once a few worms have set up shop and started breeding, the dog's immune system gets even worse at fighting them off. So humans can totally be infected with heartworm, but the reason the disease hits dogs way harder has to do with how our different immune systems react to the worms and their bacterial allies.

And that's why it's really important to talk to your vet about heartworm meds for your pets. Your furry friends will breathe a little easier and so, presumably, will you.

Well, I for one am glad that our immune systems figured out how to keep us from getting heartworms. Those parasites don't mess around, but it turns out that even parasitic worms can sometimes be good for you. I know it sounds totally wrong, but it's true. I'll hand it back to Hank to drop some knowledge.

Good Parasites
- [Hank] If your doctor told you that you were infected with worms, your first question would probably be, "How quickly can I get rid of those worms in me?" And I get that. I mean, look, you don't want worms wiggling around inside your intestines. It's enough to gross anybody out. 

But what if I told you that you might want to have parasitic worms inside you? Because some doctors have found a connection between having worms and not having immune system problems like allergies or arthritis. The idea is that these worms have set up shop in our bodies for so long, evolutionarily speaking, that our immune systems might have gotten used to them, to the point that being worm-free can actually cause its own issues.

It's part of the hygiene hypothesis, which was proposed by epidemiologists back in the 1980s to explain why allergies and autoimmune conditions like asthma are so much more common today than they used to be. According to the hypothesis, people's immune systems might be out of whack because we're too clean. Filtered water didn't exist for an awful long time, let alone hand sanitizer stations, so for much of our evolutionary history, everyone was constantly exposed to things like bacteria and parasites.

It's your immune system's job to keep these things from settling in and harming you when they get inside, so when it finds something foreign, it defends your body by triggering inflammation, that hot, red, swollen achiness. These symptoms can happen because the area is flooded with an army of white blood cells. The compounds they release either attack the foreign material or call in reinforcements.

But the compounds that do the bulk of the attack don't just target invaders. They can harm your own cells, too, and your body can get caught in the crossfire, which causes damage and pain.

Allergies, for example, are a special case of inflammation where the body is overreacting to something that's usually harmless, like pollen or dust. And autoimmune disorders come from parts of your own body triggering inflammation, like rheumatoid arthritis, where joints basically become permanently inflamed, or multiple sclerosis, where the immune system attacks the protective coating around nerves and sometimes the nerves themselves. These conditions are becoming more common these days, especially in wealthy nations where you would think easy access to high-quality medical care would prevent them.

That's where parasitic worms, collectively known as helminths, come in. This group includes things like tapeworms, nematodes, and flukes, which steal nutrients to survive. Most get cozy in another animal's intestines or blood. Some species can cause pretty severe symptoms, like the worms behind schistosomiasis, which can cause anemia, liver failure, bladder cancer, or other awful conditions, but many others don't. Like, if you had a tapeworm right now, you might have no idea, which is a super creepy thought, actually.

For those more benign species, the fallout that can come from launching your immune system nukes at them can be worse than the damage from the worms themselves, which is why some epidemiologists think that our immune systems have evolved to function with certain parasites to some extent. That might sound kind of backwards, but studies have found that rates of asthma and allergies are higher in places with fewer parasite infections, like those with more sanitation and access to healthcare.

And even though treatment is obviously worth it when the worms are causing health problems, other research has suggested that getting rid of parasites can have unintended side effects. For instance, in a 2006 study, ridding 317 children from Gabon of their intestinal parasites made some of them have an allergic reaction to mites. Similarly, a 2011 study looked at more than 2500 Ugandan women, some of whom were treated with deworming meds while they were pregnant.

While the treatment helped prevent potentially serious complications in both adults and babies, it increased the likelihood that the kids would have eczema or wheezing, both symptoms of allergic responses. And a small study of 12 multiple sclerosis patients found that those with worms had less nerve damage over time, but, when four of them were treated, their multiple sclerosis symptoms got worse.

It seems strange that having a parasite infection could keep you healthier in these specific ways, so to figure out why this pattern exists, immunologists have looked at how our bodies respond to helminth infections. They've found that some parasitic worms seem to make our immune systems kind of hold back by releasing anti-inflammatory signals that make it so our bodies don't go overboard trying to kill the parasites. At the same time, they're also reducing the inflammation that leads to autoimmune conditions and the overreaction to allergens.

Helminths could also spur the production of regulatory T cells, which recognize parts of your body that might trigger inflammation and turn down the response. T cells normally keep your immune system from staying in attack mode after the invaders are already dead or from freaking out in response to harmless stuff. like pollen, and this was seen in those 12 multiple sclerosis patients. Those with parasites had more regulatory T cells recognizing a protein that triggers the attack of neural tissue, which could be why they had less nerve damage. 

Doctors are trying to figure out what it is about the worms that triggers these regulatory mechanisms, that way they might be able to turn the compounds involved into treatments for all kinds of autoimmune diseases. It would be like the benefits of the worms without all the worms. To be clear, we here at SciShow do not recommend infecting yourself with worms to try and, like, cure your tree nut allergy, unless your doctor prescribes them, which is kind of possible. Some doctors are putting the hygiene hypothesis to the ultimate medical test, clinical trials. Most of these trials are still in the early stages, and results are mixed, but some researchers remain hopeful.

We already know that our health depends on tons of other organisms that live on and in our bodies,

so maybe parasitic worms are just part of that, just a lot bigger. But again, we are not recommending that you stop washing your hands or, like, walk around barefoot around a lot of human feces. Don't do that.

- [Michael] So there you have it. The world is full of all kinds of weird and wonderful worms, and even the bad worms aren't always bad. Like most things on this planet, worms are complex, and we should probably give them a bit more credit.

Thanks for watching this SciShow compilation. If you want to learn even more about worms, how about a whole list of reasons why nematodes are the most important animal you've ever seen? Or, if you want to help us make more videos, you can head over to Patreon.com/SciShow.

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