Biology: Diversity of Deuterostome Species

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I want to keep going with this whole coelomate idea, but let's do just a quick review of where we've been and where we're going. Remember, the point here is that we're looking at these invertebrates, this generic term, what is an invertebrate, these backbone-less animals. Even from the outside we're saying, "What does that mean?" And that's why we call it a generic term. But we've got to call them something and so we call them invertebrates. And we started to think, how are we going to divide these things off, how are we going to diversify them in terms of our own classification scheme? The first thing we've got to pull off, let's pull off the parazoa. So we pulled off the parazoa and we looked at the sponges. Then we said, "Now, let's look at the radiata," and we looked at the cnidarians. And then we said, "But now we've got to talk about the formation of the coelome and some things were acoelomic, they were the acoelomates. Some were the "kind of" coelomates, we called them the pseudocoelomates, and then the coelomates. When we got to the coelomates we said, "Wow, the coelomates, there's whole groups of developmental differences in these things." One was the protostomes, and we covered the protostomes.
Welcome to the deuterostomes, the last grouping of the coelomates. Let's quickly review, and again, you may want to click back on to those older lectures where we talked about embryological development in the animal kingdom. Quick review--remember what makes you a deuterostomes. What makes you a deuterostomes are three things. Number one, your anus will be derived from the blastopore. Now, I'll say what I've said to you all through this--if these are words that are like, "What is he talking about?" go back and look these things up and see why, in some cases, a mouth comes from a blastopore and in other cases the anus comes from the blastopore. The second thing is your enterocoelus. And lastly, your cleavage pattern is radial and indeterminate.
So all of these things being said, these three things make you a deuterostome. Well, what does that mean? What do deuterostomes look like? I think I have to start out by showing you a survey of the deuterostomes, just to show you how incredibly diverse this group is. Cows are even in here. That just strikes me because I want you to see all of the things that literally are going to be grouped together. I'm going to highlight a couple of groups that I think are so cool and in many ways bizarre, one of which happens to be the one we're in.
But let's take a look at this. Obviously, we're talking about the diversity beginning in the water, and, of course, all life started in the water. What we start to see here, as we look up at our forking phylogeny, we see some very, very interesting things. I'm going to concentrate up here later, and down here later, but I want to take a look at some of these other branches. One branch that to me is just the coolest thing, are these three right here, the lophophorates. That's not very generic is it? Lophophorates is not the kind of thing you use in conversation, but it is kind of a division with these three things. Look at these.
First of all, you have a bryozoan. The first time I saw a bryozoan I thought it was a plant. They are animals that grow on a stalk--some of them. Most of you, when you've seen a brachiopod, if you've seen a brachiopod, would think it was a clam--a mollusk. Forget it. Not even close. A phoronid, you would think, is some kind of worm. We use "worm" all the time to describe these things. Why are these three considered so close phylogenetically? Well, it's very simple. They all have lophophores. What a lophophore is is a ciliated feeding mechanism. Once again, it comes down to watch what you're observing. If you've studied any evolutionary history or any fossils, brachiopod fossils are a dime a dozen. They're in rocks every place. Most people think they're clams--no, they're probably the brachiopods. So I raise this flag of caution to you, and that's why we study classification. We have learned that these things are so close because we've studied all aspects of them.
Well, there's other things, too. A group that seems to have no significance in terms of their physicals attributes when you look at them are the pterobranchs and the echinoderms, and yet they are. Arrow worms--these are voracious predators. They call them like the tiger of zooplankton. It's a worm--but no, it's not a worm. We're talking about an invertebrate animal here that is much more highly developed than an earthworm. Acorn worms, tunicates, lancelets--invertebrates. All of these are considered deuterostomes.
I want to kind of settle in on two of these groups right now, though, because they're at opposite ends of the spectrum, and in many, many ways, their anatomy is absolutely fascinating. Let me start out with the echinoderms, and then I want to go down to this group right here, the chordates, because the chordates--well, you're going to like chordates, because you happen to be one. Let's start out talking about the echinoderms. Now, the echinoderms... First of all, what is an echinoderm? Echino means spiny skin. Now, to be an echinoderm... And here's the other thing about echinoderms. There's one now. This would belong to the group called the echinoderm, but what's strange about this is there's another one. And so you say to yourself, "Holy mackerel. How does this sea urchin and this starfish, how can they both be classified as the same thing?" And if I had a sea cucumber in my hand right now, you would think I picked up a giant dropping on the bottom of the ocean. There's a sea cucumber that we see all the time in the southern Atlantic and in the Caribbean called the "donkey dong sea cucumber." You figure out what it looks like.
Well, what does that and this and this have in common? One of the coolest things about all of these echinoderms is they have a secondary radial symmetry. They have developed a radial symmetry from a bilateral ancestor. That's very, very cool. They have secondary radial symmetry. They have an endoskeleton. Now, think about invertebrates. What are invertebrates supposed to be? They're supposed to be critters with no skeleton, right? Hello? Skeletons here. That's why these generic words make me a little nervous. Their skin covers this. And by the way, this is not spines from this particular sea urchin, but you can see what we mean by spiny skin. These are spines that are probably from a pencil urchin, and these are spines--and some spines that come out of sea urchins, like diadema, for example, are very long and thin and sharp and others are clubby like this. And a starfish, if you pick up a starfish, or if you pick up a sand dollar, another one of these things, you'll see that they have little tiny spines on them, thus the spiny skin.
If you look at this you say, "I can see the radial symmetry on the starfish with its five arms, but where is it on this?" A lot of you may have sand dollars at home, and if you have one, you may want to just stop me for a second and go and get it, because you can see some very cool stuff on a sand dollar. A sand dollar is a flattened version of the sea urchin. You can actually see in there the entire idea of the radial symmetry that is common to the echinoderms. That's what a sand dollar looks like. Their evolution has been fascinating, because if you think about it, this is a starfish with its legs closed. Literally, this is a starfish where there is closure, where there is a calcium closure in between what used to be the starfish's legs.
Boy, I could get going on echinoderm diversity and the way they're classified, and we could spend hours together, because to me they're like the most fascinating group. Just one other thing about these things--they all have one other very, very unique structure in common, and that is the water vascular system. Now, the water vascular system is absolutely astounding. Basically what it is is it is a hydraulic system for pumping water. If you were to look at a starfish and take a look inside of it, you would find a canal, a ring canal, and it's kind of like this hollow thing, and it has these literally water-filled tubes that run down the leg of the starfish, and they form this system where there's actually water pumped in to these suction cups, and the suction cups go down these grooves, and by pumping water in and pumping water out, it works almost like a plunger from a toilet, and each one of those little suction cups, when it wants to walk... And to watch a starfish walk is an amazing thing, as those podia, as they're called, suction and let go, suction and let go, suction and let go. Sea urchins have it, sea cucumbers have it. I just love echinoderms, but I have other things I want to tell you about.
The other group I want to tell you about is right here, and that's the chordates. Big--this is major. Why do I want to tell you about the chordates? I want to tell you about the chordates because they gave rise to an enormous group of organisms. But wait a minute, this is supposed to be a lecture on invertebrates, and I said, "You're a chordate." Are you an invertebrate? No. So we're not going to talk about vertebrates now. But believe it or not, on your tree exists two groups of invertebrates. I want to just tell you a little bit about them, to segue in to our discussions of the vertebrates later on.
I will never forget my first zoology course. I came across this group called urochordata and the cephalochordata. Why was I so amazed by these things? Well, just like you, I was all excited about learning about chordata. Chordata--what does chordata mean? Well, chordata means a couple of things. The first thing is it means that you have something called a notochord, not a nerve cord. Very different. Not a spinal cord. A notochord is a cartilaginous rod that runs down the length of your body. In development it's going to turn into either a spine or it's going to go away, like your spinal column. In this notochord--it persists in most of these creatures. Well, I want you to think about this. Look at this diagram here of a tunicate. Look at this photograph on my right. Does that look like something that's a cousin of yours? If I were walking along and I saw that thing growing on the edge of the ocean I'd say, "Look, it's like a sea anemone or something." We kind of demean them. They're called sea squirts. But nevertheless, do you want to know something? This thing has a notochord. But it's gone. Wait a minute. How can it have a notochord and it be gone? It turns out--and this is the key to understanding phylogeny--in its embryonic stage it loses that notochord. That's the urochordata.
The last invertebrate that we have is the cephalochordata, and that's going to show you something else that's very important in all of the chordata. Pharyngeal slits--at some point in their development they have these things called pharyngeal slits. What are pharyngeal slits? Well, they kind of look like gills. Okay, this is something called a lancelet. I know it doesn't look like much to you, but you can see a real picture. Now, this thing does not have a real active lifestyle. Yes, it can swim, but it pretty much is a burrowing organism. But the key here is what I want you to see, and what I want you to see is a dorsal nerve chord and a dorsal notochord. Now this starts to look like something that--it looks a little fishy, doesn't it. And look at those pharyngeal slits. Let me tell you, the combination of having this dorsal notochord, the pharyngeal slits, sets the stage perfectly for what's going to come next, and what's going to come next are the more advanced chordates, things like you and I, but we're not going to talk about those now.
The Evolution of Life on Earth
Deuterostomes
Diversity of Deuterostome Species Page [3 of 3]