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

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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.  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’re 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?   Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you again to Wren for sponsoring  this episode of Journey to the Microcosmos.  Wren is a website where you can calculate  your carbon emissions, then offset them by   funding projects like community tree planting  in East Africa or a project that helps prevent   wildfires in California by removing dead  and flammable trees and turning them into   biochar. We will need a lot of different  approaches to stop the climate crisis,   and this is one way that you can learn more about  your carbon contribution and take some action.  You’ll answer a few questions about your  lifestyle and they’ll also show you ways   that you can start reducing your carbon  emissions.

Of course no one can reduce   their footprint to zero, but using Wren,  you can help offset what you have left.  Once you sign up, you’ll receive updates from  the tree planting, rainforest protection,   and other projects you support. Also we’ve partnered with Wren to plant   10 additional trees for the first 100 people  who sign up using the link in our description!  Thank you so much to all of the people on the  screen right now. They are our patrons on Patreon.   They said to themselves one day, “Ya know, I think  that content about tiny tiny organisms and the   whole ecology and world that is just beneath  our view really deserves to exist.” And so,   they became our Patreon patrons, and you can  do that too at  If you want to see more from our  Master of Microscopes James Weiss,   check out Jam & Germs on Instagram or TikTok.  And if you want to see more from us, there’s  always a subscribe button somewhere nearby.