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In a 1677 letter to the Royal Society of London, the Dutch tradesman and microbiology pioneer.

Antony van Leeuwenhoek described the strange appendages he had found on a creature in a sample of rainwater. He observed that the organism was "provided with divers incredibly thin little feet, or little legs, which were moved very nimbly." We don't know exactly what organism he was describing, just that it was some sort of ciliated protozoan.

But this letter is important because the "little legs" he was describing, constitute one of the earliest descriptions of a cellular organelle. Leeuwenhoek was referring to the cilia. That name would be bestowed in 1786 by Otto Muller, who took inspiration from the word "cilium," which translates to "eyelash" in Latin.

Today, we know that cilia are a bundle of microtubules, and that the synchronized beating of motile cilia can create currents in the water. We can often see cilia in action, waving around along the edges of a single-celled eukaryote as they move around, or even crowning more complex organisms like rotifers to help them gather food. So sometimes cilia are a way to move around.

And sometimes they're a way to eat. And sometimes they're both! But sometimes, they let an organism lead the most ideal lifestyle of all: one where you just hang out and let the food come to you.

One group of ciliates that manages to strive toward that ideal are the peritrich ciliates, a diverse group of protists that contain around 1,000 species. That number making up about 1/7 of all ciliates we currently know about. Peritrich ciliates are characterized by their vase-like, or depending on where you’re from vase-like shape, which opens into an enlarged mouth-like structure called the peristome that is lined with--what else?--cilia.

One of the most well-studied peritrich ciliates are vorticella, whose cilia you can see waving around here. Their peristomes are lined with two bands of cilia: one to generate water currents and to draw food towards their mouth-y thing, and another to filter through the particles. And it turns out that all that movement is quite effective.

Scientists measuring some vorticella found that even when their cilia are only 17 micrometers long, they can bring in particles that were 450 micrometers away. Imagine if there were a delicious snack 50 feet away from you, and all you had to do was wave your arms in the air. Vorticella: truly living the dream.

We featured Vorticella on a recent episode, where we followed them in real time as they dealt with rude neighbors and other little microcosmic things. We were able to film that episode because Vorticella belong to the larger order of peritrich ciliates called the Sessilida, which--as the name suggests--are all sessile. That means for most of their life, sessilid peritrich ciliates are found attached to other surfaces, be they rock, plant, or something else.

In contrast, the rest of the peritrich ciliates are grouped together as members of the free-swimming Mobilida. Of course, while these distinctions are useful, things are not necessarily as neat as "these organisms swim" and "these organisms don't." Sessilid peritrichs live their lives in one of two distinct forms. The dominant one--the one that defines them--is their adult, sessile form, which is called the trophont stage.

The trophont sessilid is attached to a surface, like the vorticella using its long stalk to stake its place. And its body lacks any cilia except for those distinct bands around their peristome. But in their youth, sessilid peritrichs are free-swimmers called telotrochs.

In contrast to their trophont stages, these telotrochs usually don't feed, so their oral cilia are tucked away. But they do have a single band of cilia around their bodies called the trochal band, which helps them swim around until they find something to attach to. While the teletroch is generally associated with young sessilid peritrichs, a trophont can return to this stage if it needs to find a new home, like this Campanella that’s decided to leave its stalk and swim off somewhere new.

And just like sessilid peritrichs do sometimes move around, there are so-called Mobilid peritrichs with an adhesive disc that allows them to attach to things. But unlike Sessilids, mobilids never lose their trochal cilia that help them swim around. And when they swim around, they really do put the “mobile” in “mobilid”...we can barely keep this one in the frame!

The evolutionary lines connecting these two orders are vague. There have been hypotheses that mobilid peritrichs evolved from the swimming telotroch stage of the sessilid life cycle. But attempts to use molecular phylogeny to verify that prediction at a genetic level have come up with varied results.

So there's a chance that these different organisms are related, but they also may be two unrelated groups of organisms who stumbled on the same ciliated oral structure through convergent evolution. Whichever order they belong to, the peritrich ciliates that do attach to a substrate make for an interesting study. Part of it is the way they bring their own drama to what seems like a dull, sedentary life.

You might have noticed this in our real time episode, where the vorticella rapidly contracted in response to passing organisms. When this happens, a vorticella’s stalk can collapse to around 20-40% of its original length in milliseconds, a fast movement that is followed eventually by a much slower and more chill relaxation back to full length. In his own observation of Vorticella, Leeuwenhoek noted that he found this sight "mightily diverting." But the choices that peritrich ciliates make in where to set up their stalks can be “mightily diverting” as well.

Sometimes it's the typical substrates, things like plants or rocks. But sometimes, it's animals. Sometimes these interactions can become more fraught.

Mobilid peritrichs, for example, are known to infect fishes and amphibians, creating both ecological and economic challenges. But when these interactions aren’t harmful, you get something called epibiosis, where an organism lives on another organism without being parasitic. And judging by the many varied animal homes that peritrich ciliates have chosen, they’re not particularly picky, like these ciliates that have found a place on this water flea to cozy up to.

Some have been found in the gills of blue crabs. Others have been found on tardigrades. And snails.

And pelagic diatoms. And the antenna of European crayfish. The list truly goes on.

In a way that has delighted us for centuries, using those tiny little legs in their mouths and along their bodies, peritrich ciliates have managed to carve out a very specific--and some might say ideal--way of life that seems to have helped them survive just about anywhere. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. Journey to the Microcosmos is produced by Complexly.

And this is a bit of a left turn, but if you are a US citizen and want to not only keep imagining the world complexly but also have a voice in your government, check out How. To Vote In Every State. The rules for voting are different depending on where you live in the US, so we’ve made a whole series explaining how to vote in each state, as well as some advice for special cases like territories and overseas voters.

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If you want to join them, check out patreon.com/journeytomicro. If you want to see more from our Master of Microcosmos James Weiss, check out Jam & Germs on Instagram. And if you want to see more from us, there’s usually a subscribe button somewhere nearby.