Notice:  As of December 14, 2016, the MindBites Website and Service will cease its operations.  Further information can be found here.  

Hi! We show you're using Internet Explorer 6. Unfortunately, IE6 is an older browser and everything at MindBites may not work for you. We recommend upgrading (for free) to the latest version of Internet Explorer from Microsoft or Firefox from Mozilla.
Click here to read more about IE6 and why it makes sense to upgrade.

Biology: Intro to Animals: Parazoa & Radiata


Like what you see? false to watch it online or download.

You Might Also Like

About this Lesson

  • Type: Video Tutorial
  • Length: 14:13
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 153 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.

About this Author

2174 lessons

Founded in 1997, Thinkwell has succeeded in creating "next-generation" textbooks that help students learn and teachers teach. Capitalizing on the power of new technology, Thinkwell products prepare students more effectively for their coursework than any printed textbook can. Thinkwell has assembled a group of talented industry professionals who have shaped the company into the leading provider of technology-based textbooks. For more information about Thinkwell, please visit or visit Thinkwell's Video Lesson Store at

Thinkwell lessons feature a star-studded cast of outstanding university professors: Edward Burger (Pre-Algebra through...


Recent Reviews

This lesson has not been reviewed.
Please purchase the lesson to review.
This lesson has not been reviewed.
Please purchase the lesson to review.

I want you to start thinking about all those animals we've talked about and I want you to keep in mind this whole phylogenetics history of the animals and how they've changed, and what some of the qualities are that we have noted to put animals in their different groupings.
We're talking about invertebrates now. One of the biggest misconceptions that students of biology have is they think that invertebrates means something. It doesn't. It's a descriptor. Invertebrates--you learn that in third or fourth grade. Invertebrates are things without skeletons. Well, you know, by now I'm sure you know that this whole idea of an internal skeleton, even that's nebulous, as you'll see.
So the point here is we want to stick with what we've established, and what we've established is a sequential way to break the animals down into groups where hopefully they are monophylatic. Remember what that means? The point that there is one ancestor per tree--mono--or per branch of the tree. That being said, thinking of a phylogeny, remembering that we believe that protists, and probably colonial protists were the great, great, great, great, great ancestors of animals, one would think that there should be some kind of animal that is kind of like pretty disorganized, that is kind of like more of a colony than an animal. Funny you should bring that up, because there are, and that's the group that we first called the parazoa. Parazoa--sort of like an animal.
The parazoa you and I know of--well, I know of as the phylum porifera, most people know them as sponges. Surprise, surprise, surprise. Well, you're not so surprised if you've watched those other lectures, but you can amaze your friends and relatives by letting them know that those things that you can buy on shelves, unless they're like synthetic, are truly the carcasses of dead animals. They're carcasses of dead parazoan.
Well, what makes a sponge so cool? What makes it an animal? That's what makes it so cool. Well, first of all, I want you to understand that a sponge actually doesn't have systems, per se. It doesn't have a digestive system. Does it take in food? Yes. It doesn't have a circulatory system, but does it circulate the food? Yes. It doesn't have a skeletal system, but does it hold itself together? Obviously. So therefore, what we have to make it an organism, per se, is the fact that there are cells that work together. There's a cooperativity in this multi-cellular situation.
Just a couple of highlights about sponges. First of all, this sponge almost looks like it's glowing in the dark and it has a million eyes. Let me tell you what all of this means. First of all, we have openings in the sponge, and these openings are where we are going to establish a circulatory pattern. What I really want to talk about are these starlike structures here. They're not that big; they're microscopic, but we kind of enlarged them just so you get the idea. These are called "spicules." There are specialized cells in sponges that make spicules. Spicules are silica; they're literally glass--silica spicules--and they hold the sponge together. There are proteins in the sponge's body called "spongin." There are cells that make the spongin. There are cells that make the spicules. The spicules are like hanging in there and they're holding all the spongin together. Why? To create a network for these other cells that are going to be walking around in there.
What other cells? For example, there are cells called amoebacytes. Amoebacytes--they kind of like crawl through the sponge, constantly taking in microorganisms, digesting them, secreting the digestive products of those microorganisms to feed the other cells. They are almost portable stomachs. So I'm starting on the outside of the sponge, because it's a good place to start, but are you starting to feel like this is an animal? And if you're starting to feel that way, I'm making my point.
How does stuff get sucked into a sponge? Well, once again, we're going to see specialization that goes with multi-cellularity. Let's take a look at this. When things get sucked into the sponge--here's one of those openings. We call them an incurrent pore. So imagine, if you will, that the water is coming down like this and going into one of those small little holes that were in the side of the sponge. In this incurrent pore you need to create a current. You can't just allow simple osmosis and diffusion to occur here. You'll notice that there are these cells arranged like a collar around the outside of the pore. They're called "collar cells," the Latin for which is choanocyte. Choanocytes are constantly beating their flagella, causing an inflow of current, causing microorganisms to be sucked in so they can be gobbled up by the amoebacytes so they can be digested, and then how does the water get out? Well, there's a big old opening at the top of the sponge called the osculum. So you see what you've established? You've established a circulation from cooperativity between choanocytes pulling the water in and the basic structure of this particular type of sponge where the water is going out.
This is just something most of you thought you'd use in the shower, for goodness sakes. Look at this thing. This is an animal. So sponges, the very first animal; the parazoa, the sort-of animal, and phylogenetically the most ancient. Our fossil evidence shows sponge spicules dating back over 650 million years old. They were literally, as far as we can tell, the very first animals. You've got to understand something with these invertebrates, though. Fossil evidence is weak for a lot of them, because the very first invertebrates didn't have the ability to make shells and so with these we get spicules, but this next group, forget about it--fossil evidence is really hard to come by.
Well, what is the next group? I've got to tell you about this very strange group first. The reason I'm telling you about this is so you can impress your invertebrate zoologist friends, because most people never even heard of these things. In fact, they weren't even discovered until the 1960's because they used to think that they were larva, and they're called the placazoa. You guys go back to your professors and you talk to them about placazoans--guaranteed A. You can tell them I said so. A placazoan is a very simple invertebrate. Probably somewhere phylogenetically between the radiata and the next group we're going to see. It's a simple organism. I can't even draw one for you. It's like flat, it's maybe four cells thick, it's multi-flagellated, each cell is flagellated, and there's like, I think, only two known species of these things. The point is that they are motile, they are multi-cellular, and therefore, they are animals--a very simple one, perhaps, very similar to the ancestral animal.
I really want to talk to you about this next group. Remember how our tree diverges? The first divergence came when the sponges broke off of there, the parazoans. But then a second divergence came up when we got this group called the "radiata." Radiata are very cool. They radiate. They radiate from a central axis. These guys were the dominant organism back about 600 million years ago or so. Why? Because they were pretty much the only act in town at first, except for the sponges. What you have here is the beginning of tissues. You have your very first tissues when you come to the radiata. What are we talking about? Of course, we're talking about things like the jellyfish and the sea anemone and the coral and etc. A jellyfish and a sea anemone? What do they have in common? Let's take a look. We'll come back to these diagrams in a second.
First of all--and if any of these terms are new to you, you want to go back and look over the phylogeny lectures. First of all, they're diploblastic, two cell layers--diploblastic. They have one single cavity, a gastro vascular cavity that only has one opening. My students are always tickled by this, because if you think about it, that means that they only have one opening that serves as both a mouth and an anus. Think about that. The third thing is they have these specialized cells--stinging cells. And these stinging cells, and this is, by the way, where this group gets its name, the name I'm about to tell you is called "cnidarians." They used to be called "coelenterates," but now they're called cnidarians, and the reason they're called cnidarians is because they have these things called cnidocytes, which I'll tell you about in a second. They're stinging cells--cnidocytes. And they have two definite stages in their life cycle--a polyp, which is what we called a sessial stage, it sits there, and/or a medusa stage. Some of them have lost one stage or the other, or it's a very brief part of their life cycle. Here's a medusa stage, what looks to you like a jellyfish.
I've got to tell you a little bit about these things; they're so cool. First of all, let's talk about a cnidocyte. How does a jellyfish sting? Well, on its tentacles--by the way, all of these sting. Some of them sting hard and some of them you don't feel. I often take my students on a marine biology trip and they go down there and they play with the sea anemones. You can actually touch most sea anemones' tentacles and they stick to you. The reason they stick to you is they're stinging you, but it's not very toxic. Don't do that to most jellyfish; they're pretty toxic. Why? Here's what they have. See this cnidocyte? What it has in it is a structure called--there's actually a structure in there that's like harpoon, and that's called a "nematocyst." Let's see what happens here. There's a little hair on a cnidocyte called a "cnidocil." I've just got to tell you these words so I can talk to you. Inside of this thing is a structure called a "cnidae." Well, some unsuspecting fool comes swimming along, perhaps it's a little microorganism, hits the hairlike structure, and triggers an expulsion of this cnidae, and on that cnidae--are you ready for this? If this doesn't scare you, I don't know what will. It makes me very happy that jellyfish are not terrestrial. That would be a strange concept. But anyway, look what happens. This cnidae--out comes shooting this harpoon-like structure. This thing has hooks on it. This is a nasty thing. And from cellular vacuole-like structures inside, pumps a toxin, as this thing literally penetrates into your body, like a harpoon, and just pumps the stuff out. It's not a good thing. It hurts, it kills, and in fact, that's what it's for, to kill prey. All cnidarians have this thing.
One last thing--their life cycle. Their life cycle is very cool. This startled the heck out of me when I first learned this stuff. Many things that you know of as jellyfish are only jellyfish part of their lives. The jellyfish is often the sexual stage of a sort of an alternation of generations in animals. Just like alternation and generations in many other things. Plants have alternation and generations; protists have alternation and generations; animals, too, have alternation and generations. It's an evolutionary continuity. And what happens is these things will reproduce sexually, form a larva, and that larva will actually form what looks like an asexual stage, a polyp stage. See--medusa, polyp. So many jellyfish look like these little sea anemones that you see sitting on the bottom. And the polyp stage then will form buds, and this is another great thing that cnidarians can do--they can bud, and that's how a coral reef is built--bud, bud, bud, and you've got these giant reefs. And then these will eventually bud off what is in essence a larval medusa stage. So this and this are the same creature, just different stages of its life.
It doesn't get any cooler than that. Actually, it does, because there is one other group of radiata called the comb jellies or the ctenoferans, and these look a lot like jellyfish, but they kind of have combs of bioluminescent material that glows in the dark. Just think of that--two cell layers thick and those cnidarians can do all those things. Wait until you see the next group and the kinds of things they can do.
The Evolution of Life on Earth
Introduction to Animals: Parazoa and Radiata Page [3 of 3]

Embed this video on your site

Copy and paste the following snippet: