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Biology: Acoelomates, Pseudocoelomates, Coelomates


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  • Type: Video Tutorial
  • Length: 13:08
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 141 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: The Evolution of Life on Earth (34 lessons, $64.35)
Biology: Invertebrates (3 lessons, $7.92)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Let's keep going through these whole histories of animals and the highlights of some of the animals and continuing to look at it from the Phylogenetic perspective. That's why I really need you to remember that this whole idea of what is an invertebrate? An invertebrate is a generic term for things that generally don't have some kind of internal skeleton. I'm not happy with that definition as you'll see later on when we get to starfish and stuff. But anyway, all of that being said, we've already taken a look at some of the phylogeny, and we've taken off some of the groups.
The first thing we did is we said, all right, let's look at the parazoa. The parazoa were sort of animals, and we talked about the sponges. But then there was another huge divergence as animals continued to develop, and we got into the cnidarians. Remember those? They were in that group, the radiata, and so were the ctenophorans, with their radial symmetry and their very simplified cell structures.
But now we're ready to make another big jump. I want you to understand something and keep something in mind. If you know anything about some of the other major groupings of life on the planet that the animals seem to be going a little bit differently. For example, the fungi. Now, if you look at a fungus, unless you're a mycologist, generally speaking, at the microscopic level, you might be able to tell fungi apart, but the fungi, we really diversified those four divisions because of the way they reproduce. Look at what we're doing with the animals. It's very physical, it's very anatomical, it's very morphological, and it's not until we get much more diverged into the different branches that we start to look at things like reproductive behaviors and specific structures. Animals have large divergencies, for example, this next group.
Now, the next thing we want to talk about is this idea of what a coelome is, because the next divergence came with a group called the acoelomates. I'll review a term for you which I hope you'll remember. First of all, A--you know that A means without, and so this next group are going to be without coelomes. And so you're saying, "I don't remember what a coelome is." Well, it's going to get more wordy here, so you may want to go back and look at some of those phylogeny lectures on the animal kingdom. In a nutshell, a coelome is a cavity. It's a cavity that forms inside of the body and it holds organs. So you can well imagine that something without a coelome is going to be pretty simple. Why is it going to be pretty simple? Well, you don't have a cavity to hold guts, and if you don't have guts, you're pretty simple. I mean, I don't want to put down the flatworms of the world, but they're just not as complicated as say, an earthworm, which, by the way, is my example--the flatworm of an acoelomate.
And a phylum that belongs in the acoelomates is called platyhelminths. I always remembered this is high school as plat and flat, the flatworms. Several cell layers thick, and what is so important about these things is the fact that they are an early branch, fairly simple, but very common in a lot of ecosystems. In fact, many of you have probably seen one of these if you've ever looked at pond water. This is a worm--well, again--worm is such a generic word it drives me crazy sometimes. This is a flatworm. What is a worm? This is why we need a phylogeny. A worm is a squirmy, slimy thing in most people's mind. Some people, believe it or not, don't even like worms. This is a flat worm, a flattened worm-like creature, completely different from the other worms, thus platyhelminths. Acoelomate--there goes that division. Now, let's go to the next one.
Well, there are some organisms that have developed sort of body cavities, and we call them the "sort of coelomates." But in Latin "sort of" is "pseudo." And so we have the Pseudocoelomates. Now, the pseudocoelomates are a very interesting group. Here comes some vocabulary. What makes them pseudocoelomates? Didn't I just say they had a cavity? Yes, but it's not a cavity like the true coelomates. That means that the cavity--the coelome--is not derived from mesoderm. All right. If I just lost you, very quickly, mesoderm, you remember that generally speaking, when we have development we have an ectoderm, a mesoderm, and an endoderm, and the mesoderm is the middle layer. Coelomates, which we're coming to, derived their coelome from the mesoderm. So again, all of this is back in those phylogeny lectures.
Now, there are two very cool groups in the pseudocoelomates that I just want to tell you a few stories about. One is the phylum rotifera--and remember, these are the taxonomic classifications, not pseudocoelomates. Pseudocoelomate is a description. Rotifera is a phylum. So there's the rotifer phylum and the phylum called the "nematodes." If you've ever looked at pond water you would have seen probably a rotifer, and you may have made a mistake, because you may have looked at this under a microscope and thought it was some kind of protozoan--big, but nevertheless a protozoan. I've got to tell you, there are some protozoan that are bigger--one-celled organisms--that are bigger than some of these. But this is multi-cellular. You can see that this thing has many, many, many, many cells, many more than a flatworm does. And it's specialized. Remember, as we went to multi-cellularity, we needed specializations, and this guy is as specialized as you and I in many ways. First of all, you see the fact that there is a head region and a bottom region--a tail region. You'll notice that this particular rotifer can stay anchored. Rotifer means "wheel animal." Why are they called wheel animals? Right there. They've got these spinning structures at the top of their head that are ciliated and they create an inflow of water so that they can get food into their digestive tract for eating and nourishment. But this is a multi-cellular heterotrophic animal, even though it's microscopic. Not all animals are like elephants, okay?
Let me tell you just a little bit about nematodes. Some of you in high school may have done a cool nematode lab. Sometimes they call these things--I know some of the scientific supply companies sell these things to teachers and they call them "vinegar eels," and they're nematodes. And some of you may have taken soil--nematodes are unbelievably common in soil. They're microscopic worm-like creatures, but they're known as nematodes or roundworms. Again, not the same as an earthworm. They're roundworms. Some are bigger, some are macroscopic, some are microscopic, some are parasitic. There are some cool diseases--well, cool to talk about--I wouldn't want one of those diseases. But if you've ever heard of some of the diseases like--I know about so many disease because I used to be in the Peace Corp and when I lived in Africa we used to have to worry about diseases like Schistosomiasis, and elephantiasis, and one of the scariest ones was in the river. It was called "river blindness." What you would do is you'd walk in the mud and these little roundworms would crawl into your feet and they'd get into your digestive system and they'd actually get into your eye and destroy the eye tissue. Dogs get parasitic roundworms, people get parasitic roundworms. These guys have really highly evolved.
Just very briefly, think about that. What's more evolved, a parasite or its host? Well, if you think about it, the host had to come first. So I don't want you ever to think that just because roundworms are ancient they've stopped evolving. They evolve just like every other organism, and if there's a new host in the neighborhood, perhaps that's a new ecological niche that a parasite can develop into. Evolution is a work in progress. Don't ever forget that.
Very briefly, I want to get to the coelomates and tell you a little bit about a group of coelomates called protostomes. I'll tell you about one group in this lecture right now and then we'll go to another very cool group later. Coelomates--organisms with coelomes. The first group I want to tell you about is--first of all, what makes something a coelomate? Well, the coelomates are going to be subdivided and they're going to be subdivided into two groups--the protostomes--and we've all seen this already, and the deuterostomes. The deuterostomes and the protostomes--well, what's a protostome, because that's what I want to talk to you about now. We'll talk about deuterostomes later. Remember about blastopores? They get their mouth from the blastopore. Look that up if you don't know. They are what is called schizocoelous, and they have spiral and determinate cleavage. Now, if that's like another language to you, you've got to look at that phylogeny lecture. Do I sound like a nag? Sorry. There's just a vocabulary that goes with biology.
I have two groups of these protostomes that I love to talk about. One, in fact, I even like to use as bait, and the other one I like to eat. Tell me which one you think this is, the one I like to use as bait or the one I like to eat. What do you think? Obviously, we're looking at the annelids, the earthworms. And this, you dare me? What do you think, I'm crazy? Let's talk just a minute or two about the annelids. Annelid means "little ring." Annelid, as in annual, as in ring. These things were so significant in evolutionary development, because what you had here was a compartmentalized... Literally if you look at a worm inside, say, from above, it's compartmentalized. It has segments. These are the segmented worms. They include earthworms, they include leeches. Each segment is like a little body cavity. I don't know if you realize this, those lines in a worm go right through into its body, and if you were to look at one segment from another--so here's to segments, they're filled with a fluid, and each one has its own little primitive kidney called a nephridia that empties out to the outside from the next one. There's one there that empties through there like that. I mean, they are literally segmented body parts that do individual things, but they also have systems that are completely systemic. For example, a worm has an actual closed circulatory system, where it will actually have a blood vessel that moves along the top of its body, comes back, goes through a couple of structures called hearts, and actually pumps blood in a circle throughout its body, with very small capillaries throughout its skin. It breathes through its skin. It has these heart-like structures, and it even has a ventral nerve cord, a cord that comes from a pretty primitive brain and goes ventrally along its surface. Do you see what's happening here? Look at how complex we're getting. We don't have eyes yet, but we have muscle, we have circulation, we have segmentation. We have everything, almost, that you and I have. But there's a few more groups in between you and I, and you'll have to watch that in the next lecture.
The Evolution of Life on Earth
Animals: Acoelomates, Pseudocoelomates, and Coelomates Page [3 of 3]

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