Bilateria: When Mouths become Other Things (Ep. 62)

Today, we are continuing our precarious climb along the ever-changing tree of life. Once I survive the three weeks of full-time lectures, we’ll make things a little more diverse on here again. There are quite a few exciting things I want to talk about, but my brain is refusing to do more than chew up this prepared content. So, today we are working hard to reach the mollusks because you know how much I love octopuses.

Quick orientation, as always: We walked down the branch of the eukaryotes (so the stuff that isn’t bacteria and all that other small shit on the prokaryotic side of things). From there, we’ve slowly made our way to the animals, so the Animalia, from where we passed the Ctenophora, those non-jelly sea combs, and the sponges, the Porifera. We essentially ignored the Placozoa blobs, and last two times covered the Cnidarians which include sea jellies, that fucking cool Portuguese Man'o'War and one of my favorite animals: the corals. That leaves us with the Bilateria, so things you could cut into two equal-ish pieces. Not that I’m saying you should cut anything in half, of course.

One thing that sets of the Bilateria, though it might sound boring, is that they have a third germ layer. Remember when we talked about diploblasty—you know that thing where the tissue balloon called a blastula folded in on itself to create a second layer? I mentioned then that there is a thing called triploblasty, so three germ layers.

So, in the beginning (on an evolutionary and individual scale), there’s the blastula, that tissue sac like a balloon. It’s filled with liquid stuff, and then there’s a layer on the outside that keeps it all together. That folds in to make the primordial mouth and the primordial stomach.

In further evolution, things then went two ways: in very simple terms, that primordial mouth would become the actual mouth end or the ass end. If the primordial mouth turns into the actual mouth, we’ve got a protostomia. If the primordial mouth becomes the after, we’ve got deuterostomia. I remember that because in one the primordial mouth was kind of like the prototype for the actual mouth. Prototype, so protostomia. Either way, the organism now has a third germ layer, the mesoderm, and a mesoderm is a pretty big deal in developmental terms.

As always, things aren’t that simple, and the whole “protostomia turn the primordial mouth into their actual mouth” is outdated. Now, they are actually classified by their primordial mouth not becoming the after. There’s a lot of diversity in the Protostomia and their development. At least, this gives me a chance to remember the name for the process: schizocoely because they can’t make up their mind… It technically means that it’s splitting, but a mnemonic doesn’t always have to be 100%, right? Anyway…

The branch on the tree of life holding the triploplastic animals is called the Bilateria. All bilateral animals are triploplastic. All triploblastic animals are bilaterial. This means that triploblasty is an apomorphy of the bilateria. An apomorphy just means that something first happened with a certain branch on the tree of life. You know, how the eukaryotes have a nucleus to store their genetic material and the archaebacteria and bacteria don’t—that’s another apomorphy. Apomorphies are essentially the distinguishing characteristics of a group.

So, the Bilateria have a third germ layer, but they also have a second big characteristic: the Bilateria have bilateral symmetry. So, they have a left and a right half that look roughly the same, and a front and a back end. You are a bilateral organism: If you look down your body, you’ve likely got symmetry. There’s likely a leg on the left where there is a leg on the right, an ear on each side of your head. Well, and you’ve got a mouth end and an ass end. So, the Bilateria have bilateral symmetry, that’s easy enough to remember.

And no, they aren’t perfectly symmetrical, of course. Even able-bodied humans usually have differently long legs or arms or a crooked spine. That’s normal. But in the grand scheme of things, there’s bilateral symmetry, and a cranial (head) and a caudal (ass) end.

For comparison: The cnidarians had radial symmetry, so you would be able to cut pizza slices that are similar. You can’t do that with bilateral symmetry. Your leg doesn’t look the same as your arm. And then there’s your head. You only have one of those, so even if you count arms and legs the same, the symmetry is destroyed by your noggin.

The Protostomia: Mouth Turns Mouth—or Something

The Protostomia, so the ones where the primordial mouth doesn’t turn into the anus, includes two clades, the Ecdysozoa and the Spiralia. In the Spiralia, we find the Gnathifera, small spiralians with a chitin jaw, and the Lophotrochozoa, a giant group of mostly worms, but also the mollusks. So, the octopus alone would be reason to look into the Spiralia. In the Ecdysozoa, there’s also mostly worms, but then there are the arthropods, so insects and crustaceans, and some of those are pretty cool, too.

If you wanna know what distinguishes the Spiralia, they undergo a special mechanism in their embryogenesis. It’s just that when the cell divides, it does that in a more spiraling fashion as for the rest of the animals.

The Spiralia

The Spiralia all have spiral cleavage in common, an apomorphy, as we’ve learned. As always, things aren’t as clear-cut, but in the grand scheme of things, this is true. Depending on whom you ask the Spiralia are divided differently, but current consensus (if there is such a thing in taxonomy) is that they contain the Gnathifera—cod, that sounds like mosquitos to me…—and the Lophotrochozoa.

As I said, the Gnatifera aren’t mosquitos, of course, but rather small organisms with chitin jaws—so they can bite, kinda like gnats. Easy to remember. They are also called the Platyzoa in some texts. The German name for “flat” is “flach” or “platt,” so considering the Platyzoa are flat animals, that’s kinda fitting.

More interestingly, we’ve got the Lophotrochozoa. Cod, that’s a catchy name. It means wheel animal, but somehow that doesn’t make me remember that name at all. Maybe I should learn Greek and Latin or whatever this origin is.

The two characteristics of the Lophotrochozoans are the lophophores and the trochophore. So, the lopho-trocho-thing is named after it’s lopho-thing and it’s trocho-thing. Creative.

The lophophore is a long tube-shaped feeding structure with a mouth on one end that’s surrounded by a circle of tentacles. The tentacles filter-feed and move food particles into the mouth with their cilia. Cilia, as we’ve probably established, are kinda like eyelash-shaped whips or mini tentacles, depending on how you wanna look at it. Behind the mouth is the tube that leads down to the digestive tract. Depending on the type of lophotrochozoan this is part of, the lophophore might be more or less developed, more or less flexible, and more or less identifiable.

The trochophora larvae are another type of larvae. With the cnidarians, we had planula larvae. With the Lophotrochozoans, we’ve got trochophore larvae. They look essentially like something between a kid’s spinning top and a ball with two rows of fur around them. That fur is cilia again, so the little eyelash-like mini tentacle things.

Image credit: By M. Readey - Own work, CC BY-SA 3.0,

In the case of annelid worms like earth worms, this larva develops in a very interesting way: it starts growing segments at the bottom. Segment by segment, they grow into the long worms that they are.

Annelids: A Quick Chat about Worms

Speaking of annelids: there are two kinds: bristle worms and collar worms. Clitellata, collar worms are things like the earth worm, and Polychaeta, bristle worms, are worms with chitin bristles called chaetae.

Clitellata: Collar Worms

If you want to shine at your next dinner party, bore your guests with the fact that the common earth worm we all know and ignore is called Lumbricus terrestris. And, don’t get me wrong: I really think earth worms are a super important part of ecosystems, but that doesn’t mean I need to find them interesting.

And if you want to prevent people from eating anything at said dinner party, you could also tell them about the Hirudo medicinalis, the “medicinal leech,” though I’m not sure how much medicinal value it still holds today. People used to take these leeches for bloodletting. I mean, some people still believe in that stuff, of course. Why do we think it’s so unfathomable that we are very wrong about certain things these days, when we used to think it was a good idea to let leeches suck out our blood? Granted, there’s still some benefit to them medicinally, I guess because their secretions have anti-inflammatory and antimicrobial effects (well, and they are anticoagulant, but I am not even remotely surprised that an animal that sucks out blood wants it to stay nice and liquid).

Polychaeta: Bristle Worms

If I tell you that there is an animal called a sea mouse with the taxonomic name Aphrodita aculeata, you might expect something cute and cuddly. Sorry to disappoint you,  but the sea mouse is not a mouse. It’s a worm. Sea mice are bristle worms covered in a dense mat of hairlike bristles.

Image credit: By MichaelMaggs - Own work, CC BY-SA 3.0,

Granted, compared to some other polychaete worms, this one is actually pretty cute.

There’s errant bristle worms, so bristle worms that run around on and inside the ocean’s floor, and sedentary polychates that stay in place. There are cool-looking species among both types, but I definitely prefer the sedentary ones. In them, the tentacles at the end of their lophophore have developed into very pretty fans of all colors. Also, the fact that they are stuck in their tubes makes them less crittery.

But there’s one polychaete worm that I definitely want to mention here: the lug worm, Arenicola marina. Okay, it’s definitely not pretty at all. But if you ever get to tour the Wadden Sea in Europe, you’ll probably see their characteristic burrows. You’ll find a heap of sandy poop on one end, and a funnel on the other end. The worm essentially glues up an L-shape into the sand, secretions all over the walls. It then extends its head upward a bit to make the sand above its head cave in. And now, all it has to do is feed. Stuff sand in one end, poop it out in sandy heaps on the other. In areas with a lot of lug worms, the exposed sand might look like a minefield of small sandy poop heaps.

Image credit: By Original uploader was Nveitch at en.wikipedia - Transferred from en.wikipedia, CC BY-SA 3.0,

One final thing about polychaetes, and then I’ll stop talking about stupid bristle worms, I promise:

First, many of the bristle worms can actually do asexual reproduction (in addition to good old fucking) by splitting off the segments of their body. Each segment grows into a fully-fledged worm, while the original worm regenerates what they lost. Sometimes, the younger worms stay stuck to the older worm until they are ready, and the oldest worm leaves the chain.

The Ecdysozoans: A Lot More Worms, Insects, And Crustaceans

Okay, with that, we are finally done talking about the Spiralia. As we established, the other large group of the Protostomia is the Ecdysozoans, animals that molt regularly.

Their characteristics include the molting—duh—and a three-layered cuticle which makes said molting possible. The tardigrades, small organisms called water bears, have four-layered cuticles, but there’s a lot of debate on where they belong on the branches, so I’m not gonna worry about them. On a microscopic level, they are pretty cuddly looking, though.

Image credit: By Frank Fox -, CC BY-SA 3.0 de,

The Ecdysozoans are divided into the nematodes (roundworms), Mematomorpha (horsehair worms), the Scalidophora, which include something called mud dragons that looks even less like dragons than the sea mouse looks like a mouse. Well, and then there’s the Panarthropoda, the group containing the arthropods (insects and crustaceans and such), the aforementioned water bears, and yet another worm called the velvet worm.

My teacher had a giant fascination with the nematodes, but beyond a symptom called elephantiasis where some nematodes clog lymph nodes until arms, legs, and even testicles grow to elephantine sizes, I just can’t make myself care.

So, that leaves only one interesting group here: the arthropods, which we’ll look at in their own episode—just like with the mollusks.

We’ve covered a lot today, from bilateral animals to mouths becoming anuses. Mostly, we talked a lot about worms. For an episode on bilateral animals, worms really aren’t the most exciting, aren’t they?

Next time, we’ll cover the mollusks, and then we’ll look at the arthropods with all their insect and underwater “insect” diversity. Or should I consider the insects more like flying shrimp? Ah, we’ll get there.

I also want to create another series after this one with the environmental impacts of and on all of these species, so I’m excited to tell you all about how much we fucked all of this up. My favorite pastime.


I study Marine Ecology at the University of Hamburg, so a lot of this knowledge comes from hours of research and sitting through lecture after lecture.

Going through the lecture slides from school is a process that involves a shit-ton of fact-checking, as a lot of what we learn is pretty outdated. So, all semester, I google things to death, read papers and essays, ask a million questions, and discuss things with friends and classmates.

Where the source isn’t our lecture slides or unidentifiable sources from hours of late-night knowledge hunts, I have linked them in the text.

Kate Hildenbrand

Kate Hildenbrand

Kate Hildenbrand is the writer behind the essays here, author of fiction novels, the creator of the Kate Hildenbrand podcast, and a student of marine ecology. At least, that's her on the surface.