M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
When last we left our entrapment here the AP Biology student you were crying to the heavens what is a deuterostome? If only I knew I could live a fulfilled life. At least you would if you had watched part A of my talk on the animal kingdom. If you haven’t already, I really recommend that you do. It’ll make part B a lot more understandable. Plus if you do, people will think that you’re cooler and you’ll look better.
In part A, I discussed the basic characteristics of the animal kingdom, then I introduced this thing called the phyla genetic tree that is used to help organizing and make learning the different kinds of animals a lot easier.
Then I described symmetry; how radial symmetry versus bilateral symmetry that is, is used in that phyla genetic tree. Now in Part B, I’m going to continue and I actually I’m going to tell you what is the difference between a deuterostome and protostome. And then I’ll hit on the evolution of the body cavity known as the coelum and finish off by discussing the major evolutionary landmarks in various different organ systems.
You may remember that at the end of part A, I ended with a lame joke about a cliff hanger or something like that, dealing with deuterostomes, and I’ve also mentioned the word protostomes. What the heck do those words mean? If you take a look at them, deuterostome means second mouth, protostome means earlier first mouth. What’s that all about? That’s all about how do you develop your mouth and the anus and the tube that connects those two? And that’s called gastrulation.
We take a look at this. A long time ago in a galaxy not so far away called your mother’s uterus, you were this big ball of cells called a blastula. Eventually, a little tube started forming as part of it dimpled in wards and that tube grew until it eventually pokes out the other end, to form the other end of the tube, that goes from here to there.
Now if I told you as a vertebrate you’re classified as a deuterostome, can you guess which opening in you formed first? You got it, your anus. Now that doesn’t mean that all of a sudden, there is a perfectly formed anus and over here there’s perfectly formed mouth. It just means that the tube that forms the opening that first formed will eventually becomes the anus. And you and your second opening that formed will eventually becomes your mouth, with the tube here being your gut all the way through.
With protostomes, it’s the reverse. Their first opening will eventually become the mouth, and the second opening will eventually become their anus. Now is this the only difference? No, there is a big difference in what’s going on with cell division. So let’s take a look at what we do which is called radial cleavage.
If you take a look, every cell here is roughly the same size as every other cell around it. So every cell from its formation, is equal to the others. And that gives us an ability called indeterminate development. What does that mean? That means that this cell’s fate is not set yet. So if I pluck this off, and put it on here, no change. And in fact, in this early stage when I’ve gone from one cell to a few, and that’s called a big ball of cells, that’s not hollow, that’s called a malila. If I took that malila, and I separate in half, I can get identical twins. That’s very different from the protostomes. They do their differentiation which is how cells become different much earlier.
They do something called spiral cleavage. Let’s take a look at that. Here you can see, very early on, these cells are much larger than those cells and they’re offset. This offset is what gives it its name, spiral cleavage. And they, because early on, are creating differences in their cells, they show what’s called determinate development. What does that mean? That means if you have way too much time on your hands, and money, and you’re really good at techniques, you can pluck a cell of off here, shove it up there. And if this was say a fly, that fly might be born with a leg coming out of its head.
Because this was supposed to be a leg, you put on the head, and now it’s changed. When we take a look at our phyla genetic tree, let’s go ahead and do that. You can see here, there’s this big branch over here. These are all the protostomes, while these guys over here are all the deuterostomes. Now we need to go a little bit further into this. Let’s take a look at the embryonic tissues that were created in this.
There’s this hollow ball with the tube running through it. The outer layer of cells is called the ectoderm. And the ectoderm is eventually going to become things like skin, and this is a standard trick question, remember this. The ectoderm also becomes your nervous system. They always try to get you with that in a multiple choice question. Just remember that during development, in the back of your hollow bone, your gastrula, you do this weird little folding over and that created your spinal cord and eventually your brain.
If we compare that to the inner layer, that’s called the endoderm. And these are the cells lining the tube. They eventually become the cells lining the tube down your throat, and they also develop in to some of the other soft squishy organs that you have inside of you. Things like your lungs and your liver.
What about this middle layer? What do we call this middle layer? That’s called the mesoderm. Let’s take a look at that. That’s the middle layer and it becomes all the stuff in the middle. The muscles, bones and things like that. And that’s it, that’s gastrulation.
As I mentioned before, your gut is a tube that connects your mouth and your anus. Now if you’ve ever done any kind of cat or frog dissection, you know that when you open up, your gut is actually inside the space called abdominal cavity. Biologists would call that cavity or space that your gut is floating around in a coelum. Now there are some creatures that don’t even have a gut. Things like Cnidaria, the jelly fish and sea anemones.
They instead just have a common opening, one opening into a big space that doubles as both their digestive system and their convective system. That’s called a gastro vascular cavity. And that one opening that food goes in and pooh goes out, that’s called their mouth anus.
Other things that are called acoelomates have finally managed to evolve a gut, an actual tube, but with things like the platyhelmnthis. And we see one of them here. They still only have that one opening, the mouth anus. What we see with an acoelomate is they don’t have any cavity that their gut’s in. In stead we just see an outer layer of ectoderm, a middle layer of mesoderm which is neatly smack up against the endoderm that lines the hole of the gut.
Past them, other groups, specifically the nematodes and the rotifers, they developed a space called a pseudo coelum. Let’s take a look at that.
You can see here, here is a round worm, otherwise known as a nematode. And we slice it open and you can see there is that ectoderm with a lining of mesoderm creating a muscle tissue. Their gut is actually lose, floating around inside of that space. This space is called a pseudo coelum, which means literally a false cavity. I’ll get into what’s a true coelum in just a moment. If you ever dissect a nematode, you slice them open, you can pull out their guts. The only place where they’re attached is at their mouth and anus and the rest of their body just stays behind.
Let’s take a look now at what is a true coelum, because that is an evolutionary landmark that was developed by the mollusks, the annelids and the insects, or arthropods on the protostome side. On the deuterostome side, we have it in the vertebrates or chordates like us, and the echinodermata, the star fish.
A true coelum is this cavity here or herem that hasm if you notice mesoderm on both sides of this cavity. What’s the whole point of that? Well, let me take a look at that. If you notice, or you’ve got that mesoderm on both sides, that gives you muscle tissue.
Not just on your exterior that you can use for moving your body around, but you’ve also got muscle tissue and other mesodermly derived tissues lining your gut. That gives us a muscular gut. Things like nematodes, how do they move things through? They shove them all in. You can sit there and squeeze it through your oesophagus and then down and around and eventually out. And that’s using your mesodermly derived tissue. This allows, remember the mantra I’ve mentioned before, more specialization. Mention that in your AP Biology essays, you’ve got yourself another point, one more towards getting that perfect score.
Again, we have the gut in the middle lined with endoderm, with mesoderm on both sides of our true coelum and then the ectoderm on the outside. Let’s take a look at our phyla genetic tree and add in these labels. If we take a look, again we’ve got our deuterostome branch here, our protostome branch there, the platyhelminthes, remember those are the acoelomate. They don’t have a coelum. What are they doing? They’re just moving stuff through and in and out through their mouth anus.
The rotifers and nematodes, they’re both considered pseudo coelomates. What are they using their pseudo coelum for? It helps cushion them and it also helps move stuff around from their gut. They can go through the fluid that fills their pseudo coelum to the tissues that are on the exterior.
In an example of what’s called convergent evolution, we see both branches of the protostomes here and the deuterostomes there. They both have developed, in an example of convergent evolution as I said, a coelum. They’re considered coelemates. That’s it.
Now that we’re done filling up that phyla genetic tree with major developments that allowed us to assemble that tree, let’s take a look at some of the classic organ system landmarks, that the AP Biology people just love to use in their essays. And often things that are within a multiple choice question, that if you see closed circulatory system, you don’t need to bother thinking any further. You just know it’s either earthworm or a human. Let’s take a look at those.
The first big circulatory system landmark is the closed versus open circulatory system. With an open circulatory system, the blood does not always stay within blood vessels. Instead, there is some kind of hard muscular pump that pumps the fluid or blood, but it then goes out of the blood vessels and washes over the various organs in these body cavities, or sometimes they may be called a sinus. Eventually that gets suck back in and then pumped out through. It’s not very efficient, but it’s good enough if you’re a thing like an insect or an arthropod. And they’re the standard example of and open circulatory system. So remember that, arthropods, open circulatory system.
On the other hand, we vertebrates or chordate or chordata, are representatives, examples of things that have a closed circulatory system. Us and the annelids, the earthworms. So we see with that, our heart pumps, the blood goes out through arteries away form the heart, into capillaries, where it spreads out to allow movement of materials in and out. And then back in the veins. You see the blood never leaves the circulatory system with the closed system. And that’s as compared to the open system of the arthropods.
Next up is the development of the nervous systems. There is a whole spectrum ranging from the simplest of the animals like the nidera, where they basically have a loose inner network of nerve cells. And that’s called a nerve netware just random cells are communication with each other. Not very effective, but it works well enough for them.
Next up you’ll see with things like the platyhelmenthes and here we see a planeria they have what we call a nerve ladder; where we see these two nerve chords running in parallel with transverse nerves running in between them. They’re starting to get little ganglia which are collections or clusters of cells that are starting to do some processing. It’s not quite a brain, but its getting closer.
If we go up to the next things, with the annelids, the earthworms, they have an actual nerve chord but it’s running along their ventral surface; their belly.
And so if you see ventral nerve chord, it’s almost invariably going to be talking about the annelids. Sometimes they may mention that in regards to the arthropods. but it’s usually the annelids. So you can see running along their belly. they have their ventral nerve. And they’ll have bigger clusters of ganglia as compared to the flat ones, the platyhelminthes.
Now compare that to us. We’re the ones with a dorsal nerve chord. So that’s the chordata. You can see the dorsal nerve chord running along here, and despite what you may think from watching reality TV shows, we actually do have a brain. It’s not just a little cluster of cells. There is also just to toss this stuff out, you’ll see this things called a notochord that’s the cartilage rod in some of the more primitive chordata, or the actual spinal column that you and I have. We also have these things called myotomes which are the muscles, that in us are still retained between our ribs and there is your anus.
Moving on, we’ll look at how the respiratory system works. Now the respiratory system shows, again, great diversity. The simplest of those are the skin breathers which includes the simple skin breathers which are things like the nidera. The jelly fish, which just absorb through their outer layers. And then there is the complex skin breathers like the annelids. You’ll notice that they have a whole bunch of those capillaries immediately underneath their skin. And this is why earthworms have to keep their skin wet. It’s to allow them to absorb their materials through their skin.
Now compare that to the gills of fish. They have what are called internal gills. There are some salamanders that actually have external gills. Now the arthropods, which you’ll see mentioned within, is a couple of different things. Most commonly they’ll mention these things called spiracles which are tiny little holes down the sides of their body, which have little tubes called trachea, that connect to these air sacs. And that’s how they are able to get air inside of them. Some other arthropods like the spiders have these things called book lings.
The last group that I’m going to talk about is us with our lungs, where we have a complex internal system. You have your trachea here which branches into the bronchi and little bronchi called bronchioles. And finally you wind up with these little sacs called alveoli. These are the tiny sacs where we can actually collect oxygen and damp our carbon dioxide. That’s it.
Now that we’ve finished covering all that information, are you completely done? No. I would really recommend that you go to your textbook and spend some time going through and reviewing this stuff. But use this idea of making phyla genetic trees, to go through like the different kinds of vertebrates or chordata. And make up things like the branching off of the cartilage skeletons of the sharks versus the later bony skeletons of the Osteichthyes or bony fish.
You can also do this for the other kingdoms. And it’s a really affective memory technique. It’s the old idea of a picture’s worth a thousand words. So we covered a whole lot of ground, both in part A and part B today. So let’s go over this and just make sure you really got it. Remember, the whole animal kingdom is derived from our ancestor was some kind of protozoan from the Protista kingdom. Down here, towards the bottom we have our very primitive porifera or the sponges, and the nidera. The two things share radial symmetry.
Everything past that, has bilateral symmetry. Although you do remember that those echinoderms, the starfish, they do have that pentaradial symmetry in the adult. Everything else though is just bilateral symmetry all the way along. And that bilateral symmetry led to the whole thing called cephalization, the development of the head, and really amazingly good looking people.
Then we had that development of the gut that I talked about, where we have the deuterostomes, that’s us, the chordate and the starfish. And then the protostomes which are the arthropods, annelids, earthworms, Mollusca, things like clams, octopi and snails. The nematodes, round worms, rotifers, platyhelminthes the flat worms like planeria and leaches and tapeworms.
In the protostome branch we have the acoelomate platyhelminthes that means they don’t have any kind of body cavity surrounding their gut. Whereas the rotifers and nematodes, they both have that cavity called a pseudo coelum which only has mesoderm on the outer rim of the body cavity, not on the inner rim. All three of these groups and the protostomes, these two groups are in the deuterostomes, they both share that coelomate structure.
You could use this phyla genetic tree and you can add on some of the other information that we learned about the circulatory system, the respiratory system and the nervous system. And if you check your bonus material you’ll find a copy of this phyla genetic tree. You can go ahead and just write it on in. Things like the arthropods are this common example of an open circulatory system while the annelids or the chordata are the common example of a closed circulatory system.
If you do this, you’re going to do well in the test. And you can just really focus on remembering the key things that will help you spot in a multiple choice test and you’ll be able to vomit up these little factoids about these creatures to get those points that you really want to earn on the essays. Remember, on the essays you’re not trying to get a perfect score, you’re trying to get an above average score, that’s usually about 4. You’re good to go.